Castillo (7-6) took the loss during Tuesday's 4-1 defeat at the hands of the Athletics, allowing four runs on six hits and two walks with four strikeouts in 4.2 innings.
Castillo cruised through the first four frames before surrendering five consecutive two-out baserunners and all four runs in the fifth, prompting his removal. The 29-year-old fired 54 of 85 pitches for strikes with 13 of the swinging variety, though the four strikeouts match his lowest total in a start since June 15. Castillo has alternated stellar and subpar performances through four September turns, with the result being a 2.82 ERA across 22.1 innings. He's tentatively slated to pitch again Sunday in Kansas City.