Does the rotation rebound or regress in 2026?
Seattle's rotation boasted a strong reputation it didn't live up to this year. What should we expect from it in 2026?
The Mariners are a year removed from having the best rotation in baseball. But the starting staff was closer to league-average than number-one in 2025. Considering the regression we just witnessed, it’s reasonable to question whether Seattle’s “Core Five” of Luis Castillo, Logan Gilbert, George Kirby, Bryce Miller, and Bryan Woo can ever reproduce the excellence it delivered in 2024.
As always, the future is uncertain. But let’s consider two critical factors influencing my outlook for the 2026 rotation. The availability and performance of the Core Five this year.
Availability
Only two Mariners made 30-plus starts this year - Castillo and Woo. Arm-related injuries led to Gilbert, Kirby, and Miller missing significant time. In their absence, Seattle turned to Emerson Hancock, as it did in 2023 and 2024. Rookie Logan Evans also contributed with 15 starts, while Luis F. Castillo and reliever Casey Legumina covered the rest.
Starts by SEA Pitchers
Luis M. Castillo (32)
Bryan Woo (30)
Logan Gilbert (25)
George Kirby (23)
Bryce Miller (18)
Emerson Hancock (16)
Logan Evans (15)
Luis F. Castillo (2)
Casey Legumina (1)
To this nerd, the overall well-being of the Core Five is the most crucial planning factor facing the Mariners this winter. Adding intrigue to the situation, the nature of the issues sidelining starters this year and the volatility generally associated with pitcher health.
Injured SEA Starters in 2025
Bryce Miller (Elbow inflammation - twice) - 92 lost days
George Kirby (Shoulder inflammation) - 59 days
Logan Gilbert (Elbow flexor strain) - 51 days
Bryan Woo (Pectoral strain) - 10 days
Consider this. How comfortable are you knowing that two starters missed over 50 days with elbow-related injuries? Even though Gilbert and Miller could be fine next year, it’s also possible an extended IL stint lasting greater than a year looms over the horizon for either or both pitchers.
And then there’s Woo.
This year was the first time in his young career that Woo avoided an IL stint during the regular season. As a rookie in 2023, the Californian missed time with forearm inflammation. The following year, he dealt with elbow inflammation. That said, the team lost Woo in late September due to a pectoral strain, which subsequently limited his postseason availability to a pair of relief appearances in the ALCS.
In summary, the most durable Seattle starter in 2025 was Castillo, who turns 33 in a month. Injuries suffered by Gilbert and Miller could be red flags, while Woo suffered a third health setback in as many years. The totality of the situation doesn’t give me a warm and fuzzy feeling heading into the offseason. Maybe you feel differently.
Performance
A second-order effect prompted by multiple injuries was the Core Five logging 162 fewer innings than it did the season prior. But even when the quintet was available, its effectiveness noticeably regressed in multiple categories compared to 2024.
An excellent indicator of the Core Five’s decline in performance and availability was its quality start rate, which dropped 12-percent from the season prior. In 2024, Gilbert was fourth-best in baseball, while Kirby, Castillo, and Miller were top-20. A year later, the numbers were less appealing.
On the positive side, Woo’s quality start rate skyrocketed to the highest in baseball during his breakout 2025 campaign, while Castillo remained relatively stable. But the same can’t be said about the other guys.
A nine-percent reduction in Kirby’s quality start rate may not seem so bad. But he’s regressed by 13-percent since 2023. Meanwhile, Gilbert went from top-five to slightly below league-average this year.
As for Miller, just one of his 18 outings qualified as a quality start. Among 148 pitchers making at least 15 starts, only Cal Quantrill (4-percent) had a lower rate than the Texan did.
Looking ahead
My takeaway from our conversation is the Mariners could use rotation support from outside the organization. Then again, it’s possible a front office full of people more intelligent and much savvier than me feels comfortable with relying on internal options to supplement the Core Five. The most frequently mentioned candidates to fill are Hancock and Evans, plus prospects Jurrangelo Cijntje, Ryan Sloan, and Kade Anderson. But let’s do a reality check.
Hancock and Evans combined for 31 starts and 159.2 innings this year. The duo posted a 4.79 ERA with opponents producing a .265 AVG, .339 OBP, and .458 SLG against them. This stat line may be acceptable for the number-five spot in some rotations. But how comfortable would you be with both pitchers covering for one or more of the Core Five for an extended period or an entire season?
The future is bright for the youngsters, but their professional experience is extremely limited.
Cijntje pitched 108 innings during his first minor-league season after being drafted 15th overall in 2024. Sloan logged 82 frames after his second round selection the same year. Meanwhile, Anderson didn’t appear in a game with the Mariners organization after being the third overall pick in July.
Considering the inexperience of Cijntje, Sloan, and Anderson, banking on any of them to be a difference-maker at the big-league level in 2026 is aspirational. At least, that’s how I see it.
Am I painting a dark picture? You betcha. But planning for the worst while hoping for the best makes sense for a club focused on reaching its first World Series next year.
For this reason, the Mariners acquiring starters with big-league experience to address potential contingencies should be on the offseason agenda.
My Oh My…