The Marlins optioned Castano to Triple-A Jacksonville on Sunday, Christina De Nicola of MLB.com reports.
Castano would have been available out of the bullpen for Sunday's series finale with Atlanta after having not pitched since Wednesday, but the Marlins decided to send him out to the minors in favor of Joe Dunand, who was recalled from Jacksonville to provide some depth in the infield while Miguel Rojas (calf) is on the bench. Expect Castano to slot back into the Jacksonville rotation upon returning to the Triple-A affiliate.