San Antonio Spurs forward Tim Duncan underwent an MRI exam on Sunday that showed no structural damage on his sore left knee a day after he was injured in an NBA victory over Washington. Duncan, who is averaging 17.3 points and a team-high 9.7 rebounds for the Spurs, had missed four games in a row with a knee injury before returning to the court in San Antonio's 96-86 home triumph. But in the second quarter of the contest, Duncan suffered the knee injury as well as a mild sprain of his right ankle.