Right-hander Daniel Mengden had surgery Wednesday to repair a broken bone in his right foot.
He's expected to be in a walking boot for six weeks, according to an A's press release.
Mengden fractured the medial tibial sesamoid in his foot while throwing a bullpen session at home in Houston.
There is no timetable for his return to the mound, but given he'll be wearing a boot for six weeks, it's likely he'll miss all of spring training.
Mengden went 2-9 with a 6.50 ERA in 14 starts spread over two stints with Oakland last season, his first time in the majors. He was expected to start camp as part of the crowded competition for a back-end rotation spot.