Joe Saunders is traveling to Baltimore and is expected to arrive tonight. He will be in uniform on Tuesday and most likely start on Wednesday against the White Sox.
Saunders did some throwing before leaving Arizona and is "excited" to come here, said manager Buck Showalter.
The Orioles are expected to announce a corresponding roster move following tonight's game.
Showalter said left-hander Zach Britton will "probably" start Wednesday or Thursday - whichever game Saunders doesn't pitch.
"We'll get Miguel in there (the rotation) at some point," Showalter said.
Arrieta remains a strong candidate to be optioned after tonight's game.
Jason Hammel threw around 60 pitches in today's three-plus inning simulated game. He also did some fielding for the third time, again without any issues with his right knee.
Hammel is still expected to pitch Saturday for Single-A Frederick, going a maximum of five innings, before the Orioles activate him on Sept. 6.
Wilson Betemit will take batting practice today and Tuesday before heading out on a rehab assignment on Wednesday, most likely at Double-A Bowie. The Orioles expect to activate him on Sept. 1 when rosters expand.
Showalter chose to push back Chris Tillman's start until Tuesday because he didn't want Wei-Yin Chen to get too much rest. Chen is starting tonight. Also, Showalter felt like Tillman would benefit from an extra day.