The chances of him having that long NBA career are certainly debatable. The 6'8" forward has the shooting stroke and instincts that scouts love. He has a great feel for the game, which allows him to be a solid off the ball defender and excellent passer.
Where the questions of his viability as a NBA come into play revolve around his athleticism. He lacks the quickness to be able to stay in front of NBA small forwards and the size to keep power forwards away from the basket.
He looks like the classic tweener that will be a solid player to bring off the bench for a spark when a team wants to go small, is playing zone defense, or needs another shooter to space the floor.
NBADraft.net currently has him going with the last pick in the first round. They also sum up why he may not crack a starting lineup in the NBA:
"He can do a little bit of everything offensively: shoot, pass, rebound, just not one thing good enough that makes him stand out in a GMs eyes when you consider his physical and athletic limitations"
For a peak of March Madness in all its glory, check out this video of Derrick Williams amazing put back slam against Duke in the Sweet 16.