Worst Season Metrics Mark a Shift for Seattle’s Starter
Seattle Mariners right-hander George Kirby experienced the worst season of his career in 2026, posting a 4.33 earned run average (ERA) as a key contributor to a disappointing performance by the team's rotation.
Kirby’s struggles were reflected in advanced metrics that suggested his poor results were earned rather than unlucky. His expected ERA (xERA) was 4.20, indicating that the outcomes of his batted balls aligned with the quality of contact allowed. During this period, Kirby threw at least seven different pitches in each of his five major league seasons and recorded at least 80 throws of seven specific offerings in 2026.
The right-hander’s strikeout numbers fell to career-low levels, and his control slipped to only above-average status. These factors combined to create a challenging environment for Kirby, who had previously been a stable presence in the Mariners' starting lineup.
Four-Seamer Loses Edge as Velocity Increases
A significant shift occurred in Kirby’s pitch selection and effectiveness during the 2026 season. For the first time in his major league career, his four-seamer was not his best pitch by Run Value.
Instead, his sinker recorded a positive 2 Run Value, outperforming his primary fastball. The four-seamer itself underwent physical changes that impacted its utility. Compared to the previous season, Kirby’s four-seamer lost an inch of arm-side movement and approximately half an inch of vertical drop.
Despite these losses in movement, Kirby’s velocity on the pitch increased. His four-seamer sat at 96.7 mph in 2026, up from 96.1 mph in 2025. However, the increase in speed did not translate to improved results against opposing hitters.
Kirby recorded career-worst marks with his fastball in 2026. Opponents hit .251 against the pitch, while Kirby achieved a whiff rate of 23.4% and a strikeout rate of 20.4%. These figures represented the lowest points in his career for these specific metrics.
Repertoire Adjustments and Sinker Reliance
In an effort to adapt to his changing arsenal, Kirby reintroduced a cutter for the first time since 2024. He threw the pitch at a frequency of 4.2% during the season, achieving decent performance with the offering.
The reliance on other pitches became necessary as his fastball lost its previous dominance. The sinker’s positive Run Value highlighted its effectiveness relative to Kirby's other offerings during a season where his primary weapon failed to generate the expected results.