I just got confirmation from a couple of sources. Starter Brad Bergesen has been optioned to Triple-A Norfolk, and reliever Alberto Castillo will be called up to take his place.
Not sure when this will become official, but Bergesen already has been informed, and Castillo should be on his way out to the West Coast tomorrow morning.
The Orioles will be able to go with four starters for the time being because of days off Thursday and next Monday. By my calculations, the Orioles won't need to bring up a fifth starter until Saturday, May 1.