The team said Balfour -- who led the American League West champions with 24 saves and a 2.53 ERA in 2012 -- is likely to be out 4-to-6 weeks after undergoing a procedure to repair a torn meniscus.
The 35-year-old right-hander issued a statement in which he said he's optimistic he'll be ready to go by the A's April 1 season opener against Seattle.
"I feel good about getting it done now," he 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."
A's Manager Bob Melvin told MLB.com Balfour was experiencing some soreness in the knee at the end of last season.
"After the surgery, they will know how extensive it is," he said. "He's a grinder and he feels like he'll be able to rehab very quickly. He keeps himself in great shape."
Swim Week Miami Beach 2014 [PHOTOS]