Castano was optioned to the alternate training site Sunday.
The 25-year-old surrendered one unearned run on five hits with one walk and one strikeout Friday, but he'll head to the alternate site before Sunday's twin bill. Castano gave up four earned runs in his first outing of the season, but he has a 1.84 ERA, 1.64 WHIP and 8:5 K:BB in his past four appearances, so he could rejoin the Marlins for the closing stretch.