Dr. Douglas Freedberg operated Thursday. The A's said recovery time usually is four to six weeks and Balfour decided to have the procedure now with the hope of returning for the start of the season. The injury is considered minor.
Balfour said through the team that the knee bothered him late last season but wasn't considered anything serious and that the knee calmed down during the offseason. So, he went about his regular regimen over the winter.
"I didn't know what it was other than I was a little sore," Balfour said in the statement. "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."
The 35-year-old Balfour went 3-2 with a 2.53 ERA and 24 saves last season for the AL West champions.
The A's exercised their $4.5 million contract option for the Australian reliever shortly after the team lost in a five-game AL division series to the Detroit Tigers.
He made 75 appearances spanning 74 2-3 innings in 2012, his ninth year in the majors. Balfour began the season as the closer, lost the job, then earned it back again down the stretch as the A's rallied over the final 10 games to stun the Texas Rangers on the final day of the regular season.
Balfour arrived at spring training earlier this week eager to get started with a pitching staff that is nearly intact from last season.
Now, he will prepare to get going on his rehabilitation as soon as possible after the operation.
"I feel good about getting it done now," Balfour said. "It will allow me to get ready for Game 1 of the season. I knew the way I was feeling I wouldn't have been able to pitch through it all season long."
Balfour threw Wednesday and felt something in the knee.