Castro will start Wednesday against the Phillies, Roch Kubatko of MASN Sports reports.
With Chris Tillman (back) on the shelf, the Orioles will turn to Castro to make a spot start in his place. The 23-year-old has held his own as a long reliever so far this season, compiling a 3.55 ERA across 25.1 innings (15 appearances). He's made one major-league start in his career, allowing three runs on six hits across 3.1 innings last season. It's unclear if Castro or David Hess, who started one of the team's doubleheader games over the weekend, will stick in the rotation while Tillman is sidelined, but that could depend on how Castro fares in Wednesday's outing.