Matt Reynolds was one of the Arizona Diamondbacks' best relief pitchers early in the season, posting a 1.98 ERA in 27.1 innings of work.
He has not pitched since June 9, though, due to a tear of his ulnar collateral ligament.
Monday it was reported by MLB.com's Tyler Emerick that the left-hander will need Tommy John surgery.
Reynolds, who first sustained the injury in June, had been close to making a return to the D-backs' bullpen this month when discomfort in the elbow forced him to abruptly end a simulated game on Thursday.
The 28-year-old Reynolds, who was acquired via a trade with the Colorado Rockies, underwent an MRI and met with team doctors before deciding surgery would be the next step.