Castillo was optioned to Triple-A Buffalo following the first game of Saturday's doubleheader against the Rays.
Castillo served as a multi-inning reliever for the Blue Jays after joining the major-league club in mid-June, and he struck out nine while allowing two runs, four hits and two walks in seven innings over his last three outings. However, he'll head back to the minors after Thomas Hatch was recalled to start the second game of Saturday's twin bill.