Castillo is slated to start Sunday's game against the Tigers, Adam Jude of The Seattle Times reports.
Castillo and Bryce Miller worked in tandem in each of the previous three turns through the rotation, but both pitchers will receive standalone starts during the final two games of the series in Detroit. Though he currently maintains an ugly 5.53 ERA and 1.45 WHIP on the season, Castillo took some steps forward over his last two outings, covering nine innings between a start and relief appearance behind Miller while striking out nine batters and yielding two runs (one earned) on four hits and four walks. Mariners manager Dan Wilson hasn't outlined his plans for the rotation beyond this weekend, so Castillo could need another positive showing in Detroit to avoid falling back into a piggyback arrangement with Miller or shifting into a full-time bullpen role.