Castillo was recalled from Triple-A Buffalo on Thursday.
Castillo has made four appearances out of the bullpen with the Blue Jays this season and has maintained a strong 2.16 ERA, 0.96 WHIP and 10:2 K:BB across 8.1 innings. He's served as a starter at times with Buffalo this year, throwing as many as seven innings in an outing. That could make him a candidate to take over the rotation spot of Yusei Kikuchi (neck), who was placed on the injured list in the corresponding move.