The Athletics didn't immediately announce the severity of the surgery or a timetable for Outman's return.
Outman visited with Dr. James Andrews in Birmingham, Ala., on Monday. Outman was transferred to the 60-day disabled list a day earlier, signaling the club expects a lengthy recovery period.
Outman was a rare bright spot for the last-place A's this season, going 4-1 with a 3.48 ERA. He left his last start in San Diego on June 19 in the second inning with what the team called a sprained elbow.
Buck, who wasn't expected to arrive in time for Monday's game, made the A's Opening Day roster before going on the disabled list May 30 with a strained left oblique muscle. He was batting .350 during his rehab assignment, and the A's are near the bottom of the AL in several offensive categories.