A Mets official confirmed Monday that Wagner, 37, underwent an MRI examination that indicated a torn medial collateral ligament in his left elbow. The team didn't say when Wagner would have corrective surgery but rehabilitation could take up to a year.
Wagner, a left-hander, tested his throwing arm between games of the Mets' doubleheader Sunday but cut the session short because of discomfort.
Wagner has had at least 20 saves for eight consecutive seasons. He had 27 saves this season with a 2.30 ERA and 52 strikeouts over 47 innings before he went on the disabled list Aug. 5.
The Mets, who have a 2-game lead over Philadelphia in the NL East with 19 games left, have been using Luis Ayala as their closer.