The team said Thursday that the right-hander was examined last week by a team physician, who diagnosed a tear of the ulnar collateral ligament. He is seeking a second opinion before deciding on surgery.
"It is a blow, because he has a power arm and he's a guy who had success for us in that playoff run last year," Dodgers manager Don Mattingly said. "Obviously, you all saw how I used him this year, into the eighth inning, the seventh inning. I remember him pitching with the bases loaded and 2-0 and getting CarGo (Carlos Gonzalez) and Tulo (Troy Tulowitzki). He's a guy we're going to miss."
Withrow went on the disabled list and will proceed after consulting another doctor next week. Last week, Withrow was demoted to Triple-A Albuquerque. He had a 2.95 ERA in 20 appearances for the Dodgers this season, with 28 strikeouts and 18 walks in 21 1/3 innings while holding opposing hitters to a .143 batting average.
There have been 20 pitchers in 2014 that have undergone Tommy John surgery.
Mark Saxon of ESPNLosAngeles and The Associated Press contributed to this report.