Castano was optioned to the alternate training site after allowing six hits and three walks over 4.2 shutout innings during Game 1 of Tuesday's doubleheader against the Mets, Joe Frisaro of MLB.com reports.
The 25-year-old threw 23 pitches during his start Saturday, came back on two days of rest to start Tuesday's matinee and ended up one out short of qualifying for the victory. Castano has a 4.11 ERA, 1.57 WHIP and 8:8 K:BB over 15.1 innings as a fill-in starter for the Marlins.