Tatsuya Imai Stats: From NPB Strikeout King to the Astros’ Bullpen
Tatsuya Imai came to Houston last winter as the most dominant strikeout pitcher in Japan, signed for three years and $54 million, and slotted in as the Astros’ No. 2 starter. Five months into his first MLB season he is pitching in relief — and the reason shows up in one column of his stat line.
Table of Contents
Key Takeaways
- Imai went 58-45 with a 3.15 ERA and 907 strikeouts in eight seasons with the Saitama Seibu Lions, and posted a 1.92 ERA in his final NPB year.
- He led the Pacific League with 187 strikeouts in 2024, the first Seibu pitcher to win the strikeout title since Daisuke Matsuzaka in 2005.
- Through Sept. 3, 2026, he is 6-4 with a 5.35 ERA in 19 MLB games, 15 of them starts.
- The stuff has translated: 11.21 strikeouts per nine innings and a .218 opponents’ batting average. The command has not: 47 walks in 70 2/3 innings.
- Houston moved him to the bullpen July 31. He has a 2.00 ERA in three relief outings since.
Tatsuya Imai’s NPB Stats: Eight Seasons With the Seibu Lions
Imai was a first-round pick of the Saitama Seibu Lions (Pacific League, based in the Tokyo suburbs) in the 2016 draft. He was the biggest arm on the board that year: a right-hander from Kanuma, in Tochigi Prefecture, who had just pitched Sakushin Gakuin High School to the summer Koshien title, the school’s first national championship in 54 years. Koshien is Japan’s national high school tournament, a two-week August tournament that is the country’s biggest star-making stage.
He signed for 100 million yen, incentives included, and took uniform No. 11 from Takayuki Kishi, who had left Seibu for the Tohoku Rakuten Golden Eagles, a Pacific League rival, in free agency that same offseason.
The start was slow. Inflammation in his right shoulder labrum kept him on the farm team for all of his first professional year. He debuted in 2018 and won his first game June 13 against the Tokyo Yakult Swallows (Central League, based in Tokyo), allowing one run over six innings, then settled into the rotation for good.
| Year | G | W | L | CG | SHO | IP | SO | BB | ERA |
|---|---|---|---|---|---|---|---|---|---|
| 2018 | 15 | 5 | 5 | 1 | 0 | 78 2/3 | 65 | 35 | 4.81 |
| 2019 | 23 | 7 | 9 | 1 | 1 | 135 1/3 | 105 | 72 | 4.32 |
| 2020 | 19 | 3 | 4 | 0 | 0 | 61 2/3 | 44 | 52 | 6.13 |
| 2021 | 25 | 8 | 8 | 3 | 1 | 158 1/3 | 137 | 99 | 3.30 |
| 2022 | 9 | 5 | 1 | 0 | 0 | 59 2/3 | 61 | 34 | 2.41 |
| 2023 | 19 | 10 | 5 | 2 | 1 | 133 | 130 | 61 | 2.30 |
| 2024 | 25 | 10 | 8 | 2 | 1 | 173 1/3 | 187 | 70 | 2.34 |
| 2025 | 24 | 10 | 5 | 5 | 3 | 163 2/3 | 178 | 45 | 1.92 |
| Career | 159 | 58 | 45 | 14 | 7 | 963 2/3 | 907 | 468 | 3.15 |
Two things stand out to an American eye. The first is the innings total: 163 2/3 in 24 starts sounds light, but NPB plays a 143-game season and runs six-man rotations, so a workhorse there starts once a week, not every fifth day. The second is the complete games. Imai threw five of them in 2025, with three shutouts, in an era when most MLB teams go an entire season without a single complete game.
The trend line is what got him a nine-figure posting market. He won at least 10 games in each of his last three seasons and combined for a 2.18 ERA across them, and he walked away from NPB at 27 — young enough that Houston could reasonably project improvement rather than decline.
The 2024 Strikeout Title
Imai’s 187 strikeouts in 2024 led the Pacific League and gave him the first individual title of his career. No Seibu pitcher had won it since Matsuzaka in 2005, a 19-year gap.
He opened that season as Seibu’s Opening Day starter and recorded a high-quality start — six or more innings with two or fewer earned runs — in six of his first seven outings. He finished 10-8 with a 2.34 ERA in 25 starts.
The underlying number is more telling than the title: with two strikes, Imai finished hitters off 50.5% of the time, second in the league. He followed up with 178 strikeouts in 2025, including a 17-strikeout game against the Yokohama DeNA BayStars (Central League, based in Yokohama) on June 17. That broke Matsuzaka’s franchise record of 16 and tied for fifth-most in a game in NPB history.
What Makes Imai Different: A Low Slot, 99 mph and a Backward Slider
Imai is 5-foot-11 and 176 pounds, and he does not look like a power pitcher until the ball leaves his hand. He releases from below a true three-quarters slot, out of a delivery built to keep effort out of it. He wore No. 11 and then No. 48 with Seibu; in Houston he took No. 45, previously worn by Carlos Lee and Gerrit Cole.
He describes himself as a fastball-slider pitcher, and he means it literally — those two pitches account for close to 90% of everything he throws, which is closer to a late-inning reliever’s approach than a starter’s.
The Fastball
The four-seamer averages roughly 95 mph and has touched 99. What makes it play up is the release height: the ball arrives at the top of the zone on a flatter plane than hitters expect from that slot, and batters describe it as rising. Takuya Kai, catcher for the Yomiuri Giants (Central League, based in Tokyo and Japan’s most popular club), has said the ball gets on hitters faster than the arm action suggests, which is what makes the timing so hard to pick up.
The delivery was assembled by imitation. Imai studied Shohei Ohtani’s mechanics in high school, Yu Darvish’s in his fourth professional season and Kodai Senga’s from his sixth. In the 2022-23 offseason he joined a group training program run by trainer Toshiharu Konoe, built around working with a pitcher’s natural body tendencies rather than against them, and came out with a delivery that finally matched what he felt on the mound. He has credited it with removing the fatigue he used to feel at 100 pitches, which is why his velocity holds late in games.
The Slider That Moves the Wrong Way
A right-hander’s slider is supposed to break from right to left as the catcher sees it. Imai’s sometimes does the opposite, moving back toward his arm side, and hitters and coaches in Japan took to calling it a backward slider.
The more practical trait is that it does not balloon. Most sliders show a hump out of the hand before they break; Imai’s shares a trajectory with his fastball until it doesn’t, then breaks hard and slightly unpredictably out of a gyro spin. In 2023, among all NPB pitchers who threw at least 300 sliders, his 19.5% whiff rate was the best across both leagues. Opposing pitchers noticed — Atsuki Taneichi of the Chiba Lotte Marines and Shuta Ishikawa of the Fukuoka SoftBank Hawks, both Pacific League rivals, went to him to ask how he threw it.
Two Pitches, 90% of the Time
Imai leaned harder on the slider starting in 2024, raising its usage from 29.8% to 41.8%. That made him the only starter in the Pacific League building an arsenal out of two pitches each thrown more than 40% of the time.
The conventional worry about a two-pitch starter is the third time through the order. Imai inverted it: his strikeout rate climbed roughly seven percentage points from the third time through onward. And in 2025 the control caught up, too — he walked 2.47 per nine innings and led the league with three walk-free starts. That combination, swing-and-miss stuff plus newfound command, is the pitcher Houston thought it was buying.
Samurai Japan and International Play
Imai won a gold medal with Japan’s high school national team at the 2016 U-18 Asian Championship and has been in the senior Samurai Japan (Japan national baseball team) picture since 2023.
He pitched for Japan at the 2023 Asia Professional Baseball Championship, the first international event under manager Hirokazu Ibata, as one of the roster’s over-age selections. Japan swept the preliminary round against Korea, Chinese Taipei and Australia and drew Korea again in the final, where Imai started. A leadoff walk and an error in the third put him behind 2-0, and he left after four innings having allowed five hits and two runs, one earned. He got no decision; Japan rallied and won it in the 10th on a walk-off to repeat as champion.
He was one of 28 players named to the Samurai Japan Series in March 2025, a two-game exhibition set against the Netherlands at Kyocera Dome Osaka staged as a tune-up for the 2026 World Baseball Classic. Japan shut the Dutch out twice, 5-0 and 9-0.
Imai will not be at the WBC. At his introductory press conference with the Astros on Jan. 5, 2026, he ruled himself out to concentrate on his first MLB season, and he was left off Japan’s roster. His name does appear in the designated pitcher pool, the reserve list teams can draw from, which keeps the door open for future tournaments.
Tatsuya Imai’s 2026 MLB Stats With the Astros
Houston signed Imai on Jan. 2, 2026, for three years and $54 million, a deal that can reach $63 million with incentives and, unusually, carries an opt-out after every season. His 2026 salary is about $16.67 million.
He debuted March 29 against the Los Angeles Angels and got Mike Trout looking for his first MLB strikeout, then gave up four runs and was gone after 2 2/3 innings. His first win came April 4 against the Athletics on 5 2/3 scoreless innings. Six days later, against the Seattle Mariners, he issued four walks and a hit batter, threw 37 pitches and recorded one out; on April 13 he went on the 15-day injured list with fatigue in his throwing arm.
The season since has been a series of extremes. His first start back, May 12 against Seattle, was four innings and six runs, his first MLB loss. Thirteen days later he threw six no-hit, scoreless innings against the Texas Rangers — four walks, two strikeouts — and handed off a combined no-hitter the bullpen finished.
June was the stretch that made the contract look right: five starts, a 4.56 ERA, 35 strikeouts and back-to-back double-digit strikeout games. July undid it. Four starts, a 7.71 ERA, 15 walks. Three times this season he has failed to finish the first inning.
| Month | G | GS | IP | SO | BB | ERA |
|---|---|---|---|---|---|---|
| March | 1 | 1 | 2 2/3 | 4 | 4 | 13.50 |
| April | 2 | 2 | 6 | 9 | 7 | 4.50 |
| May | 3 | 3 | 14 2/3 | 10 | 7 | 5.52 |
| June | 5 | 5 | 23 2/3 | 35 | 7 | 4.56 |
| July | 4 | 4 | 11 2/3 | 14 | 15 | 7.71 |
| August | 3 | 0 | 9 | 11 | 4 | 2.00 |
| September | 1 | 0 | 3 | 5 | 3 | 6.00 |
| Season | 19 | 15 | 70 2/3 | 88 | 47 | 5.35 |
Statistics through Sept. 3, 2026.
Why Houston Moved Him to the Bullpen
On July 31, manager Joe Espada said Imai would move to the bullpen for the time being. He had made 15 starts and nothing but starts to that point, going 6-4 with a 5.83 ERA, 11.05 strikeouts per nine and 6.14 walks per nine.
The move worked immediately. In his first MLB relief appearance, Aug. 3 against the Toronto Blue Jays, he threw three perfect innings and struck out the last five hitters he faced. On Aug. 12 against the San Francisco Giants he came in as the third pitcher after opener De Los Santos allowed a run in the first, worked out of a two-out, bases-loaded jam and finished with three scoreless innings on three hits, two walks and three strikeouts in a comeback win.
The strike-throwing changed with the role. Imai averaged a 59.3% strike rate as a starter; he was at 76.5% on Aug. 3 and 61.8% on Aug. 12. Across three August relief outings he allowed two runs in nine innings for a 2.00 ERA.
The Walks Are the Whole Story
Everything about Imai’s first year comes back to free passes. He has walked 47 hitters in 70 2/3 innings, a rate approaching six per nine — against the 2.47 he posted in NPB a year earlier. That is not a small regression; it is the difference between an ace and a pitcher who cannot get through the fifth.
Part of it is the baseball. The MLB ball is slicker than NPB’s, and it has wrecked the command of Japanese pitchers before. The risk was flagged before he ever signed — the view that his first year would be rocky while he adapted to the grip was already out there — and that is more or less what happened.
The rest of the profile is intact, which is the encouraging part. He is striking out 11.21 per nine and holding hitters to a .218 average, meaning the fastball and the backward slider are missing MLB bats at the same rate they missed NPB bats. MLB.com’s pre-signing comparison was Luis Castillo of the Mariners; FanGraphs projected 26 starts, eight wins and a 4.29 ERA. Losing the rotation job knocked him off that pace on both wins and ERA.
Can Imai Start in MLB?
The honest read is that he already has the harder half solved. Strikeout stuff is the thing that usually doesn’t survive the trip — hitters lay off the chase pitch, the fastball plays down against better swing decisions — and Imai’s has survived intact. A .218 opponents’ average in a full year of MLB starts is not a fluke.
What he has to rebuild is the thing he had already rebuilt once. He walked 99 hitters in 2021 and cut that to 45 by 2025 without giving back any strikeouts; the ball changed and the gains vanished. A pitcher who has made that adjustment before is a better bet to make it again than one who has never had command at all. The August strike rates, and the way relief work has let him attack hitters, are the first real evidence it is coming back.
The two-pitch mix is the other question. It carried him in NPB partly because hitters saw him once a week; in MLB, with an opt-out looming after every season, Houston has to decide whether he is a starter who walks too many people or a very good reliever. His contract runs through 2028, he was penciled in behind Hunter Brown as the No. 2 starter in spring, and there is still a month of this season to change the answer.
