The Rockies selected Castellano's contract from Triple-A Albuquerque on Saturday.
Castellano has spent most of the season at Double-A Hartford, though he made his last appearance at Albuquerque and pitched six shutout innings while striking out seven batters and allowing three men to reach base. The 25-year-old will now get a chance to show what he can do in the majors while presumably working in a long-relief role. Jeff Criswell was optioned to Triple-A to make room for Castellano on the active roster, and Valente Bellozo was outrighted to Albuquerque to free up a 40-man spot.