Castro will start Sunday's game against the Rangers, Rich Dubroff of PressBoxonline.com reports.
The Orioles waited until the last minute, only revealing Castro as Sunday's starter less than two hours prior to first pitch. The 23-year-old was a bit more stretched out earlier this season but hasn't topped 50 pitches since early May, so he is unlikely to operate as a prototypical starting pitcher Sunday, although he hasn't pitched since a 10-pitch appearance Thursday.