Roki Sasaki and Major League Baseball: How effective would his pitching arsenal be?

Roki Sasaki is a stud. Probably.

He first attracted stateside attention in the 2023 World Baseball Classic, when he sat 100 mph on the fastball and announced his presence with authority. But Sasaki, 23,  has been putting up some bonkers numbers in Japan, with a 2.02 career ERA, and there’s a chance he comes over to Major League Baseball through the posting process this offseason. Everyone should be interested, and only some will have the international cash to step to the table — the Chiba Lotte Marines could hold him for another two years, and players under 25 are subject to international bonus pool rules — but that doesn’t mean we know everything there is to know about Sasaki as a pitcher.

To some extent, there’s always a balance between what a team knows and what it doesn’t know about a free agent. Even with a mid-career position player who has played Major League Baseball for his entire professional career, there are things the acquiring team can’t know. That’s perhaps why it’s probably true that teams get more production when they re-sign their own players than when they poach a player from another team. The assumption is that they know more about the player they already had, things like makeup and health.

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When signing a pitcher from another league, that balance is skewed toward the unknown. Not only have they been playing in an entirely different league, but they are pitchers, and pitchers are subject to more season-to-season swings in production. Pitching prospects still end up with worse outcomes than hitting prospects, and even a pitcher as exciting as Sasaki is closer to a prospect than an established major-league pitcher.

That said, Sasaki has pitched in front of pitch-tracking machines, has played at one of the highest levels in baseball outside of the big leagues, and has given us some insight into where he stands health-wise. We can follow the crumb trail to get a sense of how teams might be thinking of him during a possible posting period. His availability is not etched in stone, but maybe we can understand his talent if we use the modern tools of pitching analysis.

The stuff

Sasaki threw at the World Baseball Classic in 2023, giving us pitch movement and velocities that can help us put the physical characteristics of his pitches on the same footing as pitchers from MLB. There, he had an excellent 111 Stuff+ overall, which had him in the top 10 among starters at that tournament, and would have put him in the top 10 among qualified big-league starters this past season. He was behind Cristian Javier and Sandy Alcantara at the WBC, but ahead of Jesús Luzardo and Pablo López.

One thing we know about that tournament is that the players were amped to be there. They threw harder fastballs in shorter outings. The average starter who pitched in the WBC and then again in the big leagues over the past two years lost about five points of Stuff+ in the transition. Someone like Shota Imanaga, who had a great season for the Chicago Cubs, lost even more because he was used as a reliever. Sasaki averaged 100 mph on his fastball with great two-plane movement and by Stuff+, he had the best fastball among starters not named Shohei Ohtani in that tournament.

He’s lost some of that velocity already in Japan, averaging 98.9 mph in 2023 and then 96.9 this past season. That velocity is probably pretty important. Sasaki had 17-plus inches of induced vertical movement and 13 inches of horizontal movement in the WBC. At 98-plus mph, his fastball comps to those numbers are hard-throwing relievers like Kansas City Royals closer Lucas Erceg and New York Mets setup man Ryne Stanek, and maybe Hunter Greene among starters. At 96-plus, the comps are a little less exciting: Cleveland Guardians starter Gavin Williams and reliever Yimi García have some commonalities with their fastballs. And, according to Lance Brozdowski, Sasaki also lost a couple of inches of ride on the fastball in 2024 in Japan. It’s a really good fastball either way, but there are some indications it’s moving in the wrong direction.

His slider was an 87 mph gyro slider, meaning it’s a bullet slider without a ton of movement. That WBC slider would comp well to sliders thrown by Seattle Mariners closer Andrés Muñoz and Pittsburgh Pirates starter Mitch Keller. It was down to 83.6 mph this past season, though, and that’s below the 85 mph threshold for great gyro sliders, and now it looks more like Royals starter Brady Singer’s slider. Still an asset, but you may sense a theme here.

Splitters are hard to get a handle on in small samples, but according to this NPB pitch profiler, Sasaki’s splitter got a whiff a whopping 57 percent of the time batters swung last year (25 percent of all pitches). Only Cincinnati Reds reliever Fernando Cruz this year had a better whiff percentage in MLB, and Imanaga ended up depending immensely on his splitter with a 42.9 percent whiff rate this past season.

Sasaki’s splitter passes the eye test:

Roki Sasaki’s splitter from an NPB game early last season. This one was 91mph, and you can see the wide, four-seam grip on the closeup. He kills a remarkable amount of spin for how fast the pitch is. pic.twitter.com/u9fqQBqZTr

— Lucas (@DBITLefty) March 20, 2023

It’s top-shelf stuff from Sasaki, on par with any pitcher who has come over from Japan, even if it’s down a little.

The results

Two years ago, Sasaki had a season for the ages. In 2022, he sported a 2.02 ERA with 173 strikeouts against only 23 walks in 129 1/3 innings. Just unconscionable. He followed that up with a season that was even better by rates and strikeouts (1.78 ERA, 135 strikeouts), but shorter on innings (91) due to an oblique injury. This year, bouts of upper-body fatigue and arm soreness held him to 111 innings with a 2.35 ERA but a reduced K rate — only 129 strikeouts. This follows the trend established above, where he just had obscene stuff at age 20 in Japan, and then started to fall off that peak to some degree.

Still, if you use three-year numbers to capture both the peak and what came after, he profiles really well in a key statistic. Because NPB doesn’t have as many power hitters, it’s tough to port over things like ERA or home run rate. For example, Yoshinobu Yamamoto gave up two (two!!) homers in 171 innings in Japan in 2023 before coming over and giving up seven in 90 innings against major leaguers this year.

Instead, if we use strikeouts minus walks, we have a stat that has done a good job predicting the success of pitchers coming over from Japan. Nine pitchers have come over to start at least 15 games in MLB who had at least an 18 percent strikeout-minus-walk rate in the three NPB seasons preceding their move. There are only two pitchers on the list who count as disappointments. Nine pitchers also came over and had a strikeout-minus-walk rate under 18 percent and started 15-plus games in MLB. Hideo Nomo, Yusei Kikuchi, Hisashi Iwakuma and Kenta Maeda are the success stories on that list.

Look where Sasaki sits among his peers.

Again, though, Sasaki has fallen off some. He had a strikeout-minus-walk rate above 30 percent for two years, and then last year it fell to 21.6 percent. You could be concerned about the declining stuff and results, sure. But that 21.6 percent rate would’ve been fifth on this list, between Masahiro Tanaka and Imanaga. Still pretty good.

The eye of the beholder will have an outsized say in the negotiations. Either Sasaki is an oft-injured pitcher with already declining stuff, or he’s got some of the best stuff we’ve seen from a pitcher coming over even after that decline. Given the nature of the contract negotiations, a team that convinces Sasaki that they can help him get back to 2022 in terms of health and stuff might be the team that seals the deal.

(Photo of Roki Sasaki pitching in the World Baseball Classic on March 20, 2023: Eric Espada / Getty Images)