It be a miracle if Rush falls to the Spurs but if for some reason he does, expect an immediate impact player. He could be the Spurs answer to the aging position of the shooting guard. Not only that he is young and athletic. Guy can shoot, slash, has length, and can defend -- something Pop will love.
He does need to get stronger and needs to develop some ball handling skills. Plus he did have a major injury to his ACL but doesn't show many signs of that slowing him down.
Why the Spurs should take him? Seeing how the SG position for the Spurs is questionable and old (Barry, Finley, Manu) he could be the answer for the Spurs. But I highly doubt he would be there for the Spurs to take.