Below is what I just picked off mySA.com's blog by Jeff McDonald.  It appears that the MRI is thankfully negative.  hopefully, Timmy will be back after missing only a couple of games.

December 03, 2007
Jeff McDonald: Duncan's MRI results are in
Tim Duncan's MRI results came back Monday afternoon negative, which is positive for the Spurs and their franchise player.

There is no ligament damage in his right knee, which he injured while tangling with Portland's James Jones in Sunday afternoon's game.

The Spurs are officially listing Duncan with a sprained right ankle, though his right knee remains bruised and sore for the time being. He is being ruled out of Wednesday's game against Dallas, after which time his condition will be updated.

He could miss a game. He could miss two weeks. But either is much better than the alternative.