The severity of the injury was unknown, but Duncan was not expected to return to the game.
Duncan lay on the court clutching his knee for a few minutes after Washington forward Martell Webster rolled into the back of it while fighting for a rebound. Duncan got up on his own, but limped to the locker room with the assistance of teammates Stephen Jackson and DeJuan Blair.
Duncan, who was listed as probable with a sore left knee, returned to the lineup after missing the team's past four games.
The 14-time All-Star is averaging 17.5 points and 9.8 rebounds per game.