Castillo is expected to start Sunday's game against the Mariners, Anne Rogers of MLB.com reports.
Castillo was optioned to Triple-A Omaha on Sept. 7, and he's posted a 6.75 ERA and 1.82 WHIP in 9.1 innings over three appearances following his demotion. He tossed a scoreless inning against Iowa on Thursday, so it's not yet clear whether he'll face any restrictions during Sunday's outing.