The Seattle Mariners are exactly what everybody thought they'd be through the first half of the season. Their pitching is elite, their offense is merely average, and the result is a team in possession of the American League's final Wild Card spot with little hope of truly contending in October.
Seattle is well on its way to a finish that ownership's limited investment bought. The path to the league's second-best ERA, though, has had its obstacles.
Starter George Kirby entered the year with Cy Young aspirations but got a late start to the season due to shoulder inflammation. His return was abysmal, allowing 11 earned runs in 8.2 innings across two starts. In June, he hit his stride, and he'll enter Monday's contest against the Kansas City Royals with a 3.52 ERA over his last four outings, nearly identical to his career 3.56 mark.
One adjustment got Kirby back on track
A drastic change in Kirby's approach against right-handed hitters has defined his hot streak. He ditched his four-seam fastball almost entirely. In doing so, he's become a two-pitch pitcher against his same-handed brethren, relying more on his sinker and slider while abandoning everything else.
On June 8, Kirby mowed down the Los Angeles Angels, throwing his four-seamer 35.8% of the time compared to his sinker at 28.4%. He tossed 12 sinkers compared to four fastballs in a rocky outing against the Chicago Cubs, then doubled down in Minnesota by throwing 18 sinkers to just two fastballs to righties.
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Kirby's motivation for this adjustment might lie in his slider and its struggle to generate whiffs in 2025. He's locating the pitch well and picking up called strikes in the zone. Yet, when he's missing the zone glove side, batters aren't biting.
Improving upon his current 18.5% whiff rate on sliders (22.2% vs. righties) will be important to keep hitters off of the hard stuff. Kirby tunnels all of his pitches well from a release point perspective, although the average location of his fastball (up and away) doesn't mesh as well with the path of his slider. He still elevates his sinker more than most pitchers, but he's more willing to pound righties inside with it, and he targets the lower third of the zone much more often than his heater (20% to 4.7%, via Pitcher List).
Out of the hand, his fastball, sinker, and slider look the same. But at the batter's recognition and commitment point, the latter two are a better combination.
Kirby still has work to do
Kirby's continued evolution suggests that he'll keep looking for the right answer to optimize his arsenal. Throwing nine times as many sinkers as fastballs probably isn't that.
Relying on his sinker more often against righties is paying off. Still, there's meat left on the bones of his game. Kirby is known for his 80-grade command and a relentless pursuit of winning within the zone. He's one of the few pitchers in the sport who can consistently put any of their pitches where they want them, which makes location-based improvements more applicable.
Through seven starts, right-handed hitters are treating his fastball and sinker differently. The former is generating swings and misses at an incredible 25% clip. His 12.5% called strike percentage is well below average. Meanwhile, his sinker simply isn't missing bats (2.2% whiff rate) despite earning called strikes (27.1%).
Kirby doesn't need to punch out guys with his sinker; it's naturally going to run lower whiff rates. However, quality sinkers get hitters to make weak contact out of the zone. Kirby is throwing his in the zone over 60% of the time, a 91st-percentile mark. He stands to gain from "missing" arm side, finding more ground balls and continuing to open up his slider for strikes.
With his elite command, Kirby would then have a reason to pop his fastball inside -- rather than deferring to his sinker -- to get called strikes, adding another layer of unpredictability to his arsenal.
Against Kansas City on Monday night, Kirby will be tasked with taking down the likes of Bobby Witt Jr., Salvador Perez, and Maikel Garcia. How he utilizes his fastball and sinker could offer a glimpse into the next stage of his development.
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