Why Hitters Aren't Whiffing on Shota Imanaga's Splitter as Often in 2025

[Matthew Trueblood]

De facto Cubs ace Shota Imanaga is having a strong sophomore season, to follow what was a dazzling rookie campaign. His signature pitch isn't missing nearly as many bats as it did last year, though. What's up?

Image courtesy of © Matt Marton-Imagn Images

It would be unfair—it would be downright wrong, actually—to suggest that Shota Imanaga's splitter has been worse thus far in 2025 than it was in 2024. Based on results (not just when it ends at-bats, but whether it goes for a strike or a ball on all the others), it's been worth 1.5 runs above average per 100 thrown this year. That's identical to last season's rate. By StuffPro, Baseball Prospectus's pitch-valuation model, it's been a hair better (0.9, versus 1.0, where 0 is average and a negative number is better) than it was in his first Stateside season. By PitchPro, which gathers in the same inputs as StuffPro (release point, initial trajectory, velocity, movement, count and handedness) and then bakes in location, it's been better by an even wider margin (0.0, versus 0.3). 

However, we still have to note this: Last year, batters whiffed on 43% of their swings against Imanaga's devastating lefty splitter. This season, through six starts, that number has plunged to 30.6%. It's a major change to the primary changeup for the Japanese southpaw. To project how the rest of his season will go, we have to establish an answer to the question: Why is this happening?

Firstly, let's talk about what it isn't. There's no major change in the velocity relationship between these two pitches. If anything, meanwhile, his rising four-seam fastball and his sweeper are diverging farther from the splitter in their flight than they did last year. The red triangle here marks out the movement of his four-seamer, splitter and sweeper for 2024. The blue one shows 2025.

One risk when a pitcher creates more of a seemingly desirable movement separation between two pitches, of course, is that it's easier to spot the difference early. That would make hitters less likely to chase the offspeed or breaking ball in the pair, which would deflate whiff rate. Really, though, Imanaga is doing well at disguising the splitter, just as he did last year. Here's a 3-D representation of his arsenal (with average release points, trajectories and final locations) as seen from a right-handed batter's vantage point in 2024:

Here's the equivalent image for this year:

Maybe the latter image leaves the splitter looking a hair more detectable, but only that much, and it's not like hitters have reacted much differently to it in terms of swing decisions. They swung at 62.9% of his splitters last year; they're swinging at 61.2% of them this year. 

So, it's not velocity, it's not movement, and it's not release or early trajectory. That leaves two things that I do believe are contributing to the reduction in whiffs on the splitter for Imanaga. For one: he's throwing it more, right now. Here's how often he's thrown each of his pitches to righty batters, by season, since coming to the States.

When you throw a given pitch more often, hitters will look for it a bit more. It's hard to make a significant adjustment, from a hitter's perspective, based on a few starts and a small difference, but if teams have been monitoring this closely, the surge in splitter usage against righties this year has been large enough to impel hitters to come into games hunting Imanaga's splitter more. 

The good news with this explanation for the downshift in splitter whiffs is that we should expect to see more fastball value, in compensation for it—and indeed, we do. Batters are enjoying much better results against Imanaga's fastball so far, but their average exit velocity and launch angle are unchanged, and they're whiffing on 21.7% of their swings at it, up from 17.7%. Because they're more ready for the splitter, they're less ready for the fastball.

The other reason why hitters are finding the splitter better also comes with lots of consolation: the thing is in the zone more often, and closer to the zone even when it misses. Last season, the pitch's aggregate called strike probability (the chances of a strike being called if the batter didn't swing, across all splitters thrown) was 41%. This year, it's 45%. Here's what his locations looked like in 2024.

And here's the same image for 2025 to date.

You don't want splitters in the middle of the zone, but where he's living with it is fine. Very little hard contact will happen on a well-executed splitter there, even in the higher of the two dense red spots. Being able to land it there, meanwhile, forces hitters to either swing more or accept more called strikes when they don't. That's how he's coming out even in value, even while getting so many fewer whiffs.

Imanaga has other pitches, of course. The sweeper is doing interesting things this year, too. He'll always be defined by the fastball and the splitter, though, and despite the lost whiffs on the latter, he's enjoying an excellent start to 2025 because of his adaptability. Eventually, he might need to more consistently push that splitter down out of the zone, but if and when he does, the whiff rate will rise again. In the meantime, his 2.95 career ERA is proof of his capacity to flummox big-league hitters in myriad ways.

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