Castano was optioned to the Marlins' alternate training site Thursday, Jordan McPherson of the Miami Herald reports.
Castano was hit hard in Tuesday's loss to the Brewers and has posted a 4.72 ERA and 5:5 K:BB in 13.1 innings to begin the season. He'll now attempt to sort things out at the team's alternate training site. The Marlins haven't indicated who could take Castano's place in the rotation, but Nick Neidert could be an option to start in the near future.