The Mariners announced the diagnosis on Saturday. Montero was examined by a team doctor on Friday. The date of the operation was not yet set.
The 23-year-old, acquired in a trade with the New York Yankees prior to the 2012 season, was optioned to Triple-A Tacoma on May 22. Montero hit only .208 with three home runs and nine RBIs in 101 at-bats over 29 games. The Mariners, though, were 15-10 in games when Montero started behind the plate.
Montero hit .260 with 15 homers and 62 RBIs last year in his first full season in the majors.