"It's a clean break," Washington manager Davey Johnson said. "He's going to be out for a while."
Johnson said Werth will see a specialist to evaluate the injury.
Werth's glove got caught underneath him and his wrist bent backward as he tried to grab Placido Polanco's sinking liner. Werth stayed on the ground briefly, in obvious pain, before throwing the ball back to the infield.
He walked off holding his left wrist.
Xavier Nady replaced Werth in right field. Werth was 1 for 2 with a single in Philadelphia's 9-3 victory.
Werth missed the entire 2006 season after an injury to his left wrist that put his career in jeopardy.
He broke the wrist in spring training 2005 with the Dodgers and played 102 games that season before undergoing surgery to repair a torn ligament.