It is Roki Sasaki season.

Since news broke that the 23-year-old Japanese phenom would be jumping to the United States this winter, Sasaki has captured the headlines and imagination of the baseball world. A potential ace available for a pittancethe long-limbed pitcher is firmly on the radar and in the budgets of all 30 teams.

Only time will allow us to unravel the mysteries of Sasaki’s incredibly complex free agency, so let’s take the time to investigate him as a player.

What type of pitcher is he exactly? What pitches does Sasaki throw? How hard are they? Are there any injury or durability concerns? Who are already loose comparisons in MLB? What are reasonable expectations?

Let’s dive into the data and build a comprehensive reconnaissance report on it the most sought after arm of the offseason.

Sasaki’s first taste of professional baseball

Sasaki has been known since his amateur days when he threw a fastball 101 mph, breaking Shohei Ohtani’s record for the hardest fastball ever thrown by a Japanese high school student. Despite receiving interest from MLB teams right out of high school, Sasaki chose to play in NPB for the Chiba Lotte Marines who drafted him first overall in 2019.

The highly touted pitcher missed the 2020 season to rest his young arm, at the request of his team. In 2021, he established himself as one of the best pitchers in Japan’s top league. The following season, Sasaki solidified himself as a game-changing force, throwing a perfect game and, at one point, striking out 52 consecutive batters. Then he turned 21. He pitched twice for Japan in the World Baseball Classic, starting the team’s semifinal game against Mexico in which he dominated for 3 2/3 innings before some light hits and a blast of three points do not stain its line.

Returning to NPB, Sasaki continued his excellence in 2023 and 2024, but struggled to stay healthy, going a combined 202 innings over the two seasons.

Sasaki 2024

Although Sasaki logged 111 innings last season and pitched to a 2.35 ERA, his business was inevitably down. Most notably, his fastball averaged 1.9 mph slower than in 2023. He also missed several starts due to an unspecified arm issue, an ailment that almost certainly played a role in its reduction in speed. But Sasaki was nailed at the right time, pitching eight shutouts in his final outing of the year, a masterful nine-strikeout performance in the postseason.

Durability and injury issues

If Sasaki were a “normal” free agent, with no contract cap space, there would be a lot more debate and conversation about his lack of durability. Over four seasons in NPB, he averaged fewer than 100 innings per season. In today’s game, with starting pitchers throwing fewer innings than ever, this lack of volume is not a problem in and of itself. But it’s worth noting that Sasaki has yet to take on the burden expected of a traditional ace.

Unfortunately, arm surgery, whether elbow or shoulder, seems inevitable for Sasaki. Essentially every MLB starter throwing this hard except for 2024 NL ROY favorite Paul Skenes has gone under the knife at some point. Pitcher injuries are factored into the calculations at this point; whichever team signs, Sasaki understands this reality. But because he’s such a bargain, his durability won’t be as scrutinized as, say, Yoshinobu Yamamoto’s was a year ago.

Sasaki’s arsenal

Sasaki throws three pitches: a four-seam fastball, a splitter and a slider. He used to throw a curveball, but has abandoned that offering in recent seasons.

In 2024, Sasaki ran the heater a little less than half the time, the splitter around 28 percent, and the slider at 25 percent. This represented by far the highest slider usage of his NPB career. During his dominance in 2022, Sasaki was fast/splitter about 90% of the time. Once it arrives in the United States, the slider projects to become an even more important weapon against right-handed hitters. Yet few big league pitchers throw their splitter this frequently.

The fast ball

Despite the recent drop in velocity, Sasaki still throws incredibly hard for a starter. His “diminished” average of 96.9 mph would have tied for seventh among MLB starters last season. His 2023 average of 98.8 would have been just behind Skenes’ 98.9.

The uniqueness of Sasaki’s fastball isn’t limited to pure heat. Its heater features an extraordinarily rare combination of horizontal and vertical movements. In other words, Sasaki’s fastball stays “on plane” and travels toward right-handed hitters.

Some reasonable comparisons currently in MLB are Skenes, Hunter Greene, Spencer Arrighetti and Jeff Hoffman. Skenes is the only current big league starter whose speed compares to Sasaki’s peak before 2024. The Pirates rookie also has tons of horizontal movement, but doesn’t have quite the same carry as Sasaki. Greene has the reach and speed, but less horizontal. Arrighetti has form but is in a lower gear bracket. Hoffman is a reliever but his heater has a similar, if less impressive, shape to Sasaki’s.

All this to say: if Sasaki’s heating never returns to pre-2024 levels, it’s still weird talk. If it starts hitting 99 again…it’ll be Skenes-ian and beyond.

The separator

If Sasaki’s fastball is a unicorn, his splitter is a unicorn that breathes fire, speaks five languages, and cooks world-class risotto. There really is no such thing.

Most splitters move down and toward the pitcher’s arm side. The average right-handed MLB split this season was thrown at 86.5 miles per hour and had 10 inches of horizontal break. The Taijuan Walker splitter is a good example of what the terrain generally looks like.

According to data presented by Lance Brozdowsk of Marquee Sportsi, the Sasaki divider averaged only 3.1 inches of horizontal rupture in 2023 and 0.6 inches in 2024. In other words, the land is generally moving downward. Watching video of Sasaki’s performance against Mexico in WBC, I noticed that he seems to vary the shape of the splitter depending on the maneuverability of the hitter he’s facing. Against lefties, the pitch showed a purer downward break with some arm side run, while against righties, the pitch exhibited a noticeable amount of cut.

Here is a splitter for LHH Alex Verdugo:

Here is a splitter for RHH Randy Arozarena:

It’s simply one of the most unique pitches in the world and will be a burden for big league hitters.

The cursor

Sasaki’s slider is, all things considered, his least refined offering. He threw a lot more pitches in 2024 and will likely continue this usage pattern once he gets to MLB. One might expect Sasaki to use the pitch almost exclusively against right-handed hitters as a bid that gets away from opponents on the same side. It’s a good argument, but it will need to become more coherent.

Here’s the problem with Roki

Sasaki is a monster talent, who could soon become a true Cy Young level pitcher who will rip your soul away. It offers a rare combination of current capabilities and benefits. The only real total comparison is Skenes, who made his MLB debut last season and was almost immediately one of the best pitchers in the league (Skenes was recently announced as an NL Cy Young finalist).

Sasaki’s secondary fastball metrics are higher than Skenes’, although Skenes is a much more physical presence. There will be significant bumps and injuries ahead for Sasaki, that’s just life as a pitcher, but there’s no doubt the data backs up the hype.