Hefner's first procedure was in August 2013, which caused him to miss this season.
Hefner re-injured the ligament in a rehab start a year later, according to ESPNNewYork.com's Adam Rubin.
The first surgery was performed by Mets doctor David Altchek. Hefner's second surgery will be done by renowned surgeon Dr. James Andrews, Rubin reports.
Hefner, 28, has a career 4.65 ERA and 1.324 WHIP in two major league seasons. He split time between the bullpen and the rotation as a rookie before becoming a full-time starter in his second season.
Hefner's injury led the Mets to reconsider Harvey's rehab strategy, according to Kristie Ackert of the New York Daily News. The Mets had originally thought he may be able to return before the end of the season, but his rehab was slowed down after Hefner's injury.
- Dan Gartland