Stephen Kolek’s Pitch Mix Has Stopped Working, & His Slider Is Suffering

[Yirsandy Rodríguez]

Image courtesy of © Jay Biggerstaff-Imagn Images

Stephen Kolek owns one of the lowest strikeout rates among Kansas City starters. His K% sits at 9.9 against lefties and 10.0 against righties, second-worst in the rotation in both splits. Yet his slider still generates whiffs at a level comparable to some of the best relievers in the league. In 2026, it has produced a 42.7% whiff rate and a 34.1% strikeout rate. Isolated, it belongs in a different profile.

A pitch mix is not a collection of stuff but a sequence in which each pitch gives meaning to the next. In 2025, Kolek's sinker and changeup provided that structure. Neither dominated, but both shaped at-bats. The sinker generated ground balls (64.7% GB), while the changeup neutralized lefties (.049 AVG, 26.2% K). Together, they forced hitters to cover the lower half of the zone, opening lanes for the four-seam and slider.

In 2026, that foundation is gone. Both pitches have lost their ability to alter swing decisions. The changeup no longer punishes zone contact, and the sinker no longer consistently drives the ball into the ground.

Pitch Type	Chase% 2025	Chase% 2026	BA 2025	BA 2026
Sinker	24.7%	22.2%	.267	.304
Changeup	32.3%	29.6%	.049	.310

Without that base, the rest of the arsenal flattens. The four-seam fastball is no longer protected vertically; its ground-ball rate drops from 47.2% to 30.3%, while usage rises to 27.7%. Hitters no longer need to respect the bottom of the zone.

Pitch Type	Usage 2025	Usage 2026	Whiff% 2026	K% 2026
Sinker	24.1%	22.0%	6	6.6
Changeup	11.6%	12.5%	18.4	16.1
4-Seam	25.6%	27.7%	18.1	7.5
Slider	23.4%	16.8%	42.7	34.1

The slider remains the only elite pitch in the mix. But it no longer shapes at-bats because the pitches surrounding it aren't fooling batters. The slider's usage has declined, and its role as a finishing weapon has lessened. Kolek's start against Tampa Bay made it visible. Three runs in the second inning, 42 pitches to escape it. The problem was not execution, but duration. No at-bat was forced into early commitment.

The results by times through the order confirm the pattern:

TTO	FIP	AVG	SLG	HardHit%
1st Time	3.82	.250	.408	34.9
2nd Time	4.51	.243	.392	23.0
3rd Time	6.11	.298	.574	46.2

Each pass through the lineup strips another layer of flexibility. By the third time, hitters are no longer predicting a sequence; they are attacking isolated pitches. Without sinker or changeup-shaped counts, the four-seam becomes predictable, and the slider becomes detached. The mix stops functioning, weakening all of Kolek's offerings.

In the end, the data does not describe a decline in stuff. It describes a pitch mix that no longer forces decisions. And when that happens, the outcome is already simplified before the pitch is thrown. Kolek still has a dominant weapon, but he no longer has a working system.

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