Tillman, who has been slowed with abdominal soreness, is slated to start Friday's game against the State College of Florida, and the 24-year-old's readiness for the season has been closely monitored.
The move to put him on the DL frees up another roster spot for the Orioles to start the season, and could give them an avenue to keep left-handed pitcher T.J. McFarland in the Opening Day bullpen. The team likes McFarland, 23, a Rule 5 Draft pick, and is exploring ways to trade for him or trade another reliever to clear a spot.
Tillman has pitched in unofficial spring games, which is why the team can backdate his DL trip, and he is expected to take the mound in Baltimore on April 6 assuming he doesn't have any further setbacks.
The team's fourth starter, who is still unannounced, is expected to be right-hander Jake Arrieta, which would line him up for April 5's home opener. Should Arrieta get the nod, it would mark his third consecutive home-opening start.
Last year's Opening Day starter, Arrieta has had an impressive spring, going 1-0 with a 1.56 ERA in five games (four starts). Lefty Brian Matusz is also technically still in the running for the rotation, although he's likely to be in the bullpen to start the season.
The Orioles also reassigned outfielder Trayvon Robinson prior to Thursday's game, leaving them with 32 players still in camp.
Brittany Ghiroli is a reporter for MLB.com. Read her blog, Britt's Bird Watch, and follow her on Twitter @britt_ghiroli. This story was not subject to the approval of Major League Baseball or its clubs.