Zimmermann, who in his first year in the big leagues, pitched in 16 games and was 3-5 with a 4.63 ERA and a team-leading 92 strikeouts. There is no word on when or where the surgery will take place.
The Nationals had concerns about Zimmermann's elbow since Saturday after an MRI was taken the previous day. But before making a final conclusion, the Nationals took the MRI to Dr. James Andrews in Birmingham, Ala., and he confirmed that Zimmermann has a tear in his ulnar collateral ligament. Washington also is planning to get a third opinion from Dr. Lewis Yocum in Southern California before scheduling surgery.
"Jordan was very upset. It was one of the toughest calls I had to make," Rizzo said. "He is one of our crown jewels. He is going to work hard and be in Washington rehabbing his right arm."
Zimmermann last pitched on July 18 against the Cubs before complaining of tightness in the elbow. He was not available for comment Monday.
Bill Ladson is a reporter for MLB.com. This story was not subject to the approval of Major League Baseball or its clubs.