Baltimore interim manager Juan Samuel made the announcement Tuesday, shortly before the team optioned right-hander starter Chris Tillman to Triple-A Norfolk.
Tillman was 1-4 with a 7.92 ERA in six starts with the Orioles this season. He yielded eight runs in 2 2/3 innings Monday against Tampa Bay.
Millwood was placed on the DL on July 6 with a strained right forearm. He is 2-8 with a 5.77 ERA in 18 starts.
Samuel said Tuesday that right-hander Jeremy Guthrie will receive an extra day of rest and will start Friday against Minnesota.