The Marlins optioned Castano to Triple-A Jacksonville on Sunday, Jordan McPherson of the Miami Herald reports.
After making one-inning relief appearances Friday and Saturday, Castano was unlikely to be available out of the bullpen for at least another day or two, so the Marlins chose to send him to the minors in favor of a fresher arm. Miami recalled Paul Campbell in a corresponding move, and he's expected to be available for Sunday's series finale against the Phillies as a multi-inning option out of the bullpen.