Castro was demoted to Triple-A Albuquerque on Monday.
Castro has a big arm (95.8 mph average fastball) and has done well for a 21-year-old at Coors Field, posting a 3.75 ERA and 1.08 WHIP over 12 innings. Unfortunately, the Rockies needed to option somebody to make room for lefty Boone Logan's return to the bullpen. Castro is still a part of the future in Colorado, and he should find himself back with the big club at some point before the season ends.