The Mariners outrighted Castillo to Triple-A Tacoma on Monday after he cleared waivers.
After he had been one of Seattle's top relievers since he was acquired from the Rays at the 2021 trade deadline, Castillo struggled mightily over the first month of the 2023 campaign, logging a 6.23 ERA, 1.62 WHIP and 7:7 K:BB in 8.2 innings before he was optioned to Triple-A on Saturday. Because Castillo is due $2.95 million in 2023 and has one more year of arbitration remaining, none of MLB's other 29 organizations were willing to put in a waiver claim for the 29-year-old right-hander. Though he no longer possesses a spot on the 40-man roster, Castillo could end up getting another look with the big club this season if he's able to right the ship at Tacoma.