Nelson's an interesting guy. But if you examine his most recent teams (the Mavs and the Warriors) how much he concentrates on offense and so little he concentrates on defense, his judgment is highly questionable, especially in light of the defensive-minded Celtics dominating the offensive-minded Lakers in the Finals.
Regardless, I've always been a great believer in youth. Had this kid stayed in school, he'd just be coming out now and would probably have been one of the top picks. Give him a chance. The Celtics need to take low-risk, high-reward opportunities like this given their limited financial resources.