He injured the wrist on Sunday and was hitting .213 with 10 homers and 46 RBIs in 97 games.
Oakland selected first baseman Daric Barton from Triple-A Sacramento to take Reddick's place on the roster. Brandon Moss will start in right field in Monday night's game against the Detroit Tigers and Barton will start at first base.
The Athletics also designated right-handed relief pitcher Pat Neshek for assignment on Monday. He was 2-1 with a 3.58 ERA in 41 games.
Right-handed pitcher Evan Scribner was recalled from Sacramento to take Neshek's roster spot.