?Is a fastball alone enough to carry a rotation? The Mariners’ right‑hander George Kirby endured the toughest campaign of his career, posting a 4.33 ERA that dragged Seattle’s prized staff down. His 4.20 xERA confirms every ounce of the struggle, while strikeouts sank to a career low and his famed control slipped to merely above‑average.
Seattle’s starters usually flaunt deep repertoires and constantly tweak grips, but Kirby sits at the epicenter of that experiment. Baseball Savant logs him using at least seven distinct pitches each season, and in 2026 he threw seven offerings over 80 times.
The sprawling mix offers many suspects, yet the answer may be simple: his four‑seam fastball, long his go‑to weapon, simply wasn’t effective this year.
Kirby burst onto the scene in 2022 relying almost entirely on an elite fastball that posted a Run Value of 17, ranking 14th league‑wide behind Edwin Diaz’s slider and Carlos Rodón’s four‑seamer. He leaned on that pitch 45.7% of the time, with six others never exceeding a 14% usage rate.
The four‑seamer stayed his workhorse in 2023, delivering a 13 Run Value in his first full season. It dipped to a 3 Run Value the next year, then rose to 7 in 2025, while batters managed just .188 against it and it generated a whiff rate north of 30%—a solid mark for any fastball.
For the first time in the majors, Kirby’s sinker eclipsed his four‑seamer in Run Value in 2026, posting a 2 rating. Still, the fastball lagged on several fronts, recording career‑worst opponent BA (.251), a 23.4% whiff rate, and a 20.4% strikeout rate.
The pitch also lost an inch of arm‑side movement and half an inch of vertical drop, even as velocity climbed to 96.7 mph from 96.1 mph the year before. Those shifts may have made the fastball look more like a typical arm‑slot offering, giving hitters a clearer target.
Kirby isn’t likely finished tweaking his arsenal; he re‑added a cutter for the first time since 2024, though it appeared on just 4.2% of his pitches and performed respectably. In my view, fixing the four‑seamer should be his top priority heading into 2027.
Photos courtesy of Joe Nicholson and Brian Fluharty, Imagn Images. Will Kirby’s revamped mix finally restore his fastball’s dominance in the coming season?