Oakland closer Grant Balfour has a torn meniscus in his right knee and will miss four to six weeks, reports CBS Sports. Balfour felt discomfort in the knee towards the end of last season, but the extent of the injury was unknown at the time; a recent MRI revealed the tear.
Balfour will undergo arthroscopic surgery on Thursday afternoon, reports the San Francisco Chronicle's Susan Slusser:
Balfour is expected to recover in time for the start of the season, but in the event that he is not ready to go, Ryan Cook and Sean Doolittle would be potential options to close.
Balfour, 34, had 24 saves with a 2.53 ERA and .924 WHIP with the A's last season.