Oakland Athletics reliever Grant Balfour is expected to miss 4-to-6 weeks after having right knee surgery to repair a torn meniscus on Thursday.

Given the timeline, the club is hopeful Balfour will be available in time for Opening Day.

Balfour experienced some knee soreness near the end of 2012, a season in which he went 3-2 with a team-leading 24 saves and a 2.53 ERA. The right-hander wasn't worried because the soreness went away during the offseason, but an MRI exam revealed the tear.