HOUSTON — As of May 2026, Seattle Mariners pitcher Luis Castillo has a 1-5 record with a 5.90 ERA in 11 games, prompting the team to move him from the starting rotation to the bullpen. Castillo made his first career relief appearance on May 19, 2026, against the Chicago White Sox.

The Mariners had recently used a piggybacking strategy, pairing Castillo’s starts with reliever Bryce Miller to manage innings and maintain effectiveness. That approach has now evolved into a full transition to relief duties for Castillo, whose performance in 2026 marks a contrast to his role in the 2025 season.

Castillo helped the Mariners reach the American League Championship Series for the first time in franchise history during the 2025 season. The Mariners acquired him in July 2022 when the Cincinnati Reds traded him to Seattle in exchange for a four-player package headlined by utility player Noelvi Marte.

Prior to that trade, interest in Castillo had emerged from other teams. Former Twins executive Thad Levine revealed on the 'Rosters to Rings' podcast that he had expressed interest in acquiring Castillo before the 2022 trade deadline but was rebuffed by Reds president of baseball operations Nick Krall. "The other thing I’ll share, I believe we also started the conversation with you expressing interest in Luis Castillo," Levine said to Krall, "to which you very kindly said to me, ‘you don’t have possibly enough in your whole organization to get Luis Castillo.’ To which I said, I also would like to express interest in Tyler Mahle, to which you said, ‘now, that’s a conversation we can have.’"

The Reds ultimately traded Castillo to the Mariners later in the 2022 season, where he became a key part of Seattle’s starting rotation through the 2025 campaign.