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Castillo allowed two hits and issued two walks while striking out six batters over four scoreless innings in a no-decision against the Athletics on Monday.
After struggling mightily as a traditional starter this season, Castillo was deployed out of the bullpen for the first time in his career in his previous appearance May 19. The veteran righty was then moved back into a starting role Monday, but the plan all along was to pair him with Bryce Miller. Castillo ended up thriving with that setup, holding the A's to just two hits while racking up 12 whiffs and six punchouts over four frames. He was pulled after 68 pitches in favor of Miller, who fared well himself in closing out the contest by giving up two runs over five innings. It remains to be seen how Seattle will use Castillo going forward, but he made a case to remain a starter -- at least as part of a tandem -- with his performance Monday. Of course, it will dampen his fantasy value considerably if he's limited to fewer than five innings and 70 pitches as he was against the A's.
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Mariners' Luis Castillo: Starting in pairing with Miller
Rotowire
Mariners manager Dan Wilson said that Castillo will start Monday's game against the Athletics in Sacramento and will be used as part of a tandem with Bryce Miller, Ryan Divish of The Seattle Times reports.
In his last appearance Tuesday against the White Sox, Castillo pitched in relief for the first time in his 10-year big-league career, covering 2.1 frames and allowing two earned runs behind Miller, who started and worked the first 5.2 innings. The two will be deployed in the opposite order for the upcoming turn through the rotation, and while Castillo should have a higher innings floor as a starter, his poor form of late could result in Wilson leaning on Miller to handle the larger share of the pitching workload. Though the Mariners are taking a fluid approach to the back end of the rotation, Castillo may need a positive showing against the Athletics to avoid a demotion into a more traditional bullpen role. He'll head into Monday's start with a 6.41 ERA and 1.55 WHIP on the season, with both ratios being the worst marks of his career by a considerable margin.
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Mariners' Luis Castillo: Struggles persist in relief outing
Rotowire
Castillo (1-5) allowed two runs on a hit, two walks and a hit batsman while striking out four over 2.1 innings, taking the loss Tuesday versus the White Sox.
Castillo was fine through two innings, but manager Dan Wilson left him in for the start of the ninth. That ended up being a mistake, and Andres Munoz failed to stop the White Sox's rally after coming in for mop-up duty. Castillo's poor relief outing stands in contrast to a superb start from Bryce Miller, who appears to have bumped Castillo out of the rotation. Castillo is at a 6.41 ERA, 1.55 WHIP and 47:18 K:BB through 46.1 innings over 10 outings (nine starts). It'll likely take an injury to another pitcher for Castillo to get back in the rotation at this point.
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Mariners' Luis Castillo: Piggybacking Miller on Tuesday
Rotowire
Castillo is slated to piggyback starting pitcher Bryce Miller during Tuesday's game against the White Sox, Adam Jude of The Seattle Times reports.
Castillo will be making the first relief appearance of his MLB career Tuesday after going 1-4 with a 6.34 ERA and 1.57 WHIP through his first nine starts of the season. Since Miller covered 5.1 innings in his 2026 Mariners debut last week in Houston following his return from the injured list, manager Dan Wilson is likely to let Miller work as long as he's effective against the White Sox. With that in mind, Castillo's innings projection for Tuesday is highly variable, and how he and Miller both perform could determine how Seattle sets up its rotation moving forward. If Miller comes through with a strong outing Tuesday, he'll likely draw another start the next time the fifth spot in the rotation comes up, while Castillo could be thrust into a more flexible bullpen role on a longer-term basis.
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Mariners' Luis Castillo: Finally gets into win column
Rotowire
Castillo (1-4) earned the win Thursday against the Astros, allowing three runs on four hits and three walks in 5.2 innings. He struck out six.
Castillo was working on a bounce-back start Thursday, giving up a lone run over his first five frames, but the Astros got to him for two more runs in the sixth inning to make his line more middling. The Mariners might have left the 33-year-old right-hander in the game for a bit too long considering that the hurler tossed a season-high 108 pitches. Castillo has now yielded at least three runs in three straight outings and six of his last eight, posting a disappointing 6.34 ERA, 1.57 WHIP and 43:16 K:BB over 44 innings in 2026. He's tentatively lined up to piggyback with Bryce Miller early next week versus the White Sox, which would mark Castillo's first career relief appearance if he works as the secondary pitcher.
Castillo (0-4) was tagged with the loss Saturday against the White Sox, allowing four runs on five hits across four frames. He struck out six.
This loss against Chicago ballooned Castillo's woeful ERA to 6.57 across 38.1 innings in eight outings this year. He's lost his past three starts and has given up at least four runs in each of those games. Castillo has been confirmed to start Thursday against the Astros, but according to Ryan Divish of The Seattle Times, after that the Mariners plan to piggyback him with Bryce Miller in the rematch series against the White Sox that runs May 18-20.
Mariners' Luis Castillo: Could get bumped from rotation
Rotowire
The Mariners could remove Castillo from the rotation when Bryce Miller (oblique) returns next week, Adam Jude of The Seattle Times reports.
It's just one option the team is mulling, as Miller and Emerson Hancock are also being considered for the bullpen or piggyback roles. It would be an easy decision to bump Castillo from the rotation if it were based purely on performance, as he's posted a 6.29 ERA, 1.66 WHIP and 31:13 K:BB over 34.1 innings in his first seven starts. However, Castillo has never pitched in relief at the major-league level, while Miller and Hancock both have. A six-man rotation is also under consideration. Castillo will start Saturday in Chicago against the White Sox in what could be his final opportunity to convince the Mariners he should remain in the rotation.
Castillo (0-3) allowed four runs on six hits, two walks and a hit-by-pitch while striking out five over six innings to take the loss Sunday versus the Royals.
Castillo was doing fine until the fourth inning, when he allowed four straight batters to reach base to open the inning. Three of them came around to score, and the Royals tacked on an extra run in the sixth. Castillo has allowed at least four runs in five of his seven starts this season, though this is the third time he's allowed four earned runs. He has a 6.29 ERA, 1.66 WHIP and 31:13 K:BB through 34.1 innings. Until Castillo is able to limit baserunners better, he is likely to continue having trouble preventing runs. His next start is projected to be on the road versus the White Sox.
Castillo (0-2) took the loss Monday, allowing seven runs on seven hits and two walks over five innings against Minnesota. He struck out three.
Castillo worked a clean first inning but struggled from there, allowing four runs in the third inning, highlighted by a three-run homer from Kody Clemens. Known for his consistency with a career 3.61 ERA and just one full season above 4.00, Castillo is off to a rough start in 2026, posting a 6.35 ERA and 1.73 WHIP with opponents hitting .317 across 28.1 innings (six starts). He'll look to get back on track in his next outing, which is tentatively scheduled for Sunday against Kansas City.
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