According to Roch Kubatko at MASN Sports, starting rotation hopeful Steve Johnson is suffering from a sore right latissimus dorsi and hasn't thrown since March 22nd. It's unclear at this time if Johnson will be ready when the season starts.
Although it hasn't been officially announced, all signs point to Jake Arrieta being named the fifth starter in the rotation when the Orioles break camp in a few days. Johnson's injury makes that even more likely since he was one of the pitchers competing with Arrieta for the spot.
As Johnson is quoted in Kubatko's article, his soreness might be a minor thing but at this point it's best to let hit heal totally rather than the possibility of it lingering all season. Johnson was probably slated for AAA anyway, so hopefully he can take the proper time to heal and then be ready to go if and when the Orioles need him.
Speaking of pitchers with injuries, Kubatko also mentions in his article that Chris Tillman will be the #4 starter depending upon his status. Tillman has been suffering from a sore abdomen that doesn't appear to be serious. He's slated to pitch against the Manatees on Friday in the Orioles annual community college match up.
Word on the Twitter is that if Tillman is ready to go, he'll pitch the fourth game of the season, which is the home opener. But if he's not ready, they'll place him on the DL with a back date so that he can come off in time to pitch the fifth game of the season. This seems a little convoluted to me as they could just swap his place with the #5 starter if he only needs one more day without putting him on the DL. But we all know how much Dan Duquette loves his merry go round, and maybe he'd prefer to have another player on the bench for the first series if he can get away with it.