Mariners right-hander George Kirby just put together the worst season of his career. He stumbled to a 4.33 ERA as one of the key contributors to a disappointing season for Seattle’s vaunted rotation. Kirby’s 4.20 xERA suggests he earned every bit of the poor performance. His strikeout numbers fell to career-low levels while his elite control slipped to only above-average.
Seattle’s starters have typically boasted expansive arsenals. They also tend to tinker, adding and subtracting pitches and adjusting shapes to get different results. Kirby is at the center of it. Baseball Savant has him throwing at least seven different pitches in each of his five big-league seasons. He threw seven offerings at least 80 times in 2026.
The wide arsenal gives plenty of candidates to point to for Kirby’s down year, but the answer might be a simple one. His four-seamer, which has long been his best pitch, wasn’t good enough this season.
Kirby burst onto the scene as a rookie in 2022 almost exclusively due to an elite fastball. The pitch racked up a Run Value of 17 that season. It ranked 14th among all pitches, just behind Edwin Diaz’s slider and Carlos Rodon’s four-seamer. Kirby leaned on his fastball 45.7% of the time. He threw six other pitches, but none with a usage rate above 14%.
The four-seamer continued to carry Kirby as he established himself as a top-of-the-rotation option. It recorded a 13 Run Value in 2023, Kirby’s first full season in the majors. The heater regressed a bit to a 3 Run Value the following season, before rebounding to a 7 Run Value in 2025. Opponents hit just .188 against Kirby’s four-seamer last season. It posted a whiff rate above 30%, which is a solid mark for any pitch, but especially a fastball.
For the first time in the majors, Kirby’s four-seamer wasn’t his best pitch by Run Value in 2026. That would be his sinker at a 2 Run Value. Of course, Run Value isn’t the only way to measure a pitch’s performance, and Kirby’s four-seamer struggled by multiple metrics. Kirby notched career-worst marks in opponent batting average (.251), whiff rate (23.4%), and strikeout rate (20.4%) with the fastball this season.
The characteristics of Kirby’s four-seamer also changed this year. It lost an inch of arm-side movement and about a half-inch of vertical drop compared to last season. Notably, he also threw it harder than ever. Kirby sat at 96.7 mph with the fastball, up from 96.1 mph in 2025. The movement profile of the pitch has shifted at times during Kirby’s career, so it’s not necessarily a red flag. The results on contact could mean the four-seamer’s combo of velocity and movement in 2026 looked more like what hitters would expect from a pitcher with Kirby’s arm slot, making it easier to square up.
Kirby likely isn’t done tinkering with his arsenal. He reintroduced a third fastball this season, bringing back the cutter for the first time since 2024. Kirby only threw it at a 4.2% clip, but it performed decently well. His repertoire will likely look different once again in 2027. Fixing the four-seamer should be the primary goal.
Photos courtesy of Joe Nicholson and Brian Fluharty, Imagn Images
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