Colorado optioned Castano to Triple-A Albuquerque on Tuesday.
Castano made five appearances with the big-league club since being called up Sept. 1. Through 6.2 innings, the 28-year-old posted a 2.70 ERA, 1.20 WHIP and 4:3 K:BB. The Rockies reinstated Kyle Freeland (neck), forcing the team to remove Castano from the active roster.