Atkins, who made seven relief appearances the last two seasons for the Chicago Cubs, will start Tuesday night at Texas.
The Orioles also are putting outfielder Luke Scott on the 15-day disabled list. Scott, hitting .223 with nine home runs, has been playing through a right labrum tear. He last played Sunday.
Baltimore needed a starting pitcher after Alfredo Simon, who had been a candidate to pitch the middle game of the series, was instead needed to pitch 4 1/3 innings in relief Monday night. Baltimore starter Chris Jakubauskas gave up six runs while facing only 13 batters in the 13-4 loss to the Rangers.