The Rockies optioned Castellano to Triple-A Albuquerque on Monday.
Castellano picked up a save in Sunday's 23-9 blowout win over the Athletics with three scoreless frames in his major-league debut. He's clearing off the roster to make room for the return of Victor Vodnik (elbow) from the injured list. Castellano is likely to be used as a starting pitcher and/or bulk reliever at Albuquerque.