Last week, I covered how the Royals changed Ryan Bergert from a guy way out-performing his peripherals to one with significantly better peripherals to the point that even as he improved in total, he was underperforming the new ones. This week, I wanted to take a look at what they did with Stephen Kolek.
The raw numbers were, without a doubt, quite impressive. In his time with San Diego, he accrued a 4.18 ERA with an atrocious 8.9% K-BB% in 79.2 innings over 14 starts. When he finally got to Kansas
City, he pitched to a 1.91 ERA with a still-bad, but much-improved 12.8% K-BB% in 33 innings over only five starts. And he might not even make the Royals’ rotation out of Spring Training next year!
Let’s dive into the deeper numbers, shall we?
Well, I gotta admit, that wasn’t at all what I was expecting to see.
The first thing that jumps out to me is that he drastically reduced the usage of his sinker, which was easily his highest-graded pitch prior to the trade. None of his pitches got whiffs while he was in San Diego, but at least the sinker got chase and poor expected results for hitters. Instead, he led the way with his four-seam fastball in Kansas City, which grades exceptionally poorly. It also got the worst chase of its type other than his sweeper, which he barely threw, and his worst whiff rate of any pitch except the sweeper and cutter – which he threw only marginally more often than the sweeper – and by far his worst expected outcomes.
I’m starting to see why his SIERA of 4.56 with San Diego only improved marginally to 3.91 with Kansas City.
Interestingly, the sinker was still a really strong weapon for him when he did throw it and became the only pitch, in either sample, which could generate whiffs at an above-average rate for its type.
But the thing that really stands out after the increased fastball usage is his changeup. He more than doubled its rate of usage from 8.8% before the trade to 19.3% after, and with good reason, as it became his second-best pitch by both grade and tjStuff+. Like the rest of his pitches, it didn’t generate chase or whiff compared to other changeups, but the xwOBA was exceptionally good. The average velocity ticked up almost 1 MPH, but generally, you’d expect a faster changeup to be worse, unless the fastballs also got faster, but they didn’t. The movement and spin rate numbers are all pretty similar, too, so it doesn’t seem like the Royals got him to change his grip or anything. The one glaring difference is that he went from throwing it in the strike zone 19.4% of the time to putting it in there 40.9% of the time.
In fact, that’s the biggest takeaway from his arsenal overall. In a drastic difference from Bergert, he didn’t change any of his pitches, and he actually got less chase, less whiff, and had a worse xwOBA for the Royals. But he threw ~5% more strikes total, primarily by throwing his non-fastballs in the zone way more often. The one commonality between Kolek and Bergert is that they threw their sweepers more often and for strikes more often. That’s absolutely something to keep an eye on with the Royals going forward for all of their pitchers, but also to see if Kolek throws his sweeper even more often in 2026.
I initially was going to skip looking at his pitch plots because all of his pitches showed so much similarity in their movement profiles before. But I glanced at them before submitting this and realized that, individually, they didn’t change much, But in the aggregate you can see some things happening.
On the right side, you can see that the slight changes in his sinker and changeup have them overlapping less often. Some of you may recall that I advocated against Brady Singer adding a changeup to his arsenal (I thought he needed something that moved on a different axis) because the movement profile of a circle change – the most common changeup in MLB today – and a sinker can be so similar. Separating them even just a little, however, may have done Kolek quite a bit of good and may explain why his sinker and changeup each performed better in KC. He also got a bit more separation between his fastball and cutter. I’m not really sure what to make of the difference in the shape of the location of his sweeper, but it’s something I’ll continue watching in 2026, should he remain with Kansas City.
Kolek had a terrific, if short, debut for KC. He was especially impressive in his ability to pitch deep into games when the rest of the staff was running on fumes. Looking at this data, I probably won’t be campaigning for him to get that rotation spot out of Spring Training next year. But as a guy making close to the minimum who can step in and fill in for an injured starter, he should still be an incredibly valuable piece for the Royals next season. And it will be interesting to see if he does start throwing different pitches or getting different movement on them after working with Kansas City’s coaching staff for a full offseason.
