Reddick left Saturday's 7-6 victory against Miami in the fourth inning with a strained right knee. He had an MRI on Sunday morning, but the A's had not seen the results when they decided to put him on the DL.
"Until we get the MRI we're not sure, but regardless it was the feeling of the training staff that he wouldn't be available," manager Bob Melvin said before Sunday's game at the Marlins. "He's still sore and stiff. I think it scared him pretty good last night, but he felt better than he thought he would feel this morning."
Melvin said Reddick will not be on the field doing any activities for at least five days.
Reddick had just been activated on Tuesday after he spent three weeks on the disabled list with a hyperextended right knee. In four starts since coming back from his first DL stint, Reddick had gone 5 for 11 with three RBIs. He is batting .229 with four homers and 24 RBIs on the year.
Oakland also promoted first baseman Nate Freiman from Triple-A Sacramento, and he got the start in the series finale in Miami.
Freiman hit a three-run homer in his 2014 debut as the A's beat the Marlins 4-3 to complete a three-game sweep.
The Associated Press contributed to this report.