Tillman allowed two unearned runs in a career-high 8 1/3 innings in his return to the majors.
The Orioles don't want Tillman being idle through the All-Star break. He'll start Monday night for Bowie and rejoin the club next weekend after his mandatory 10 days have expired.
It's technically a demotion, since he's going back down to the minors, but it's only being done to keep him on schedule. That's all.
Tillman has earned a spot among the starting five, which includes Jason Hammel, Wei-Yin Chen and Jake Arrieta. Manager Buck Showalter is expected to decide between Dana Eveland and Miguel Gonzalez for Friday's assignment in Anaheim.
Tillman hasn't pitched in the Eastern League since 2008.
As for a corresponding roster move, Showalter and executive vice president Dan Duquette must decide whether to go back to a four-man bench or bring up a reliever.