The Orioles promoted righty Tommy Hunter from Triple-A Norfolk to start against Philadelphia and sent righty reliever Miguel Gonzalez to their top minor league affiliate.
Manager Buck Showalter announced the reshuffling of the pitching staff. The Orioles have lost nine of 12 games.
Arrieta was pulled from the rotation after losing his sixth straight decision. The Phillies tagged him for career highs of nine runs and 11 hits in four innings Friday night, leaving him at 2-8 with a 6.32 ERA.
"Going to the bullpen is going to do a lot of things for me. It's going to allow me to get a lot of the thoughts and emotions out of my head. When I come into the game—just pitch," Arrieta said.
"My thought process has been way too in depth. I've got way too much going on in my head right now, trying to tweak so many things mechanically instead of going out there and feeling comfortable and executing pitches," he said.
Following the loss to the Phillies, Arrieta acknowledged: "I'm just kind of lost right now."
Showalter said the 26-year-old Arrieta has all the right physical attributes to be successful. In the middle of his struggles against the Phillies, Arrieta struck out four straight batters.
Showalter said the Orioles wanted to give themselves their best chance to win and "Jake as a starter isn't it right now."
"We'll see how it works out," Showalter said.
Arrieta was 6-6 as a rookie in 2010 and went 10-8 with a 5.05 ERA last year. He pitched seven scoreless innings against Minnesota on opening day this year and later threw eight shutout innings at Yankee Stadium.
The Orioles had already planned to recall Hunter to face the Phillies. The right-hander is 2-3 with a 5.59 ERA in 10 starts for Baltimore this season.
Showalter said the team waited until 5 minutes before the deadline for the roster move in deciding to send down Gonzalez and keep Arrieta in the majors.
Gonzalez made his big league debut May 29. The righty was 0-0 with a 2.45 ERA in two games.