Grant Balfour has undergone arthroscopic surgery on his right knee to repair a torn meniscus.
The Athletics said the recovery time is usually four-to-six weeks and Balfour decided to have the procedure now with the hope of returning for the start of the season.
"I didn't know what it was other than I was a little sore," Balfour said. "Obviously, we were in a pennant race so I was going to pitch. ... I was able to work out with very little soreness, so I really wasn't concerned."
Balfour went 3-2 with a 2.53 ERA and 24 saves last season for Oakland.