I don't think anyone expects Willie to guard smaller players full time, and even if he could, then who is the player that should be guarding them guarding. Where Willie is valuable is in switches where he ends up on a Chris Paul etc. Does that mean Paul can't get past him? Of course not, but it's a hell of lot more difficult than getting past Cousins or Koufos. I saw almost every game Willie played at Kentucky, and basically he does what is asked of him, and in my opinion, sometimes that was too much. There were games when he was asked to guard the best player on the other team, which many times left him out on the perimeter, and away from the basket. Difficult to rebound from out there.
From a coaches point of view, Willie looks capable of being all things to all people. But maybe he would be at his best if asked to play a more traditional role as a PF or a Center, as you suggest. He's more than capable of doing that. But up to now, he's been jerked around as a plug the hole player without a defined role per say. Hopefully Joerger can remedy that. At Kentucky Willie was a terrific pick and roll defender, and there's no reason that can't translate to the NBA. It's rare for a seven footer to criticized for not being able to guard the Chris Paul's and Lillards of the world fulltime, But in a way, it's a compliment to Willies abilities.
