How can anyone speculate about his value without more info? Apparently the issue stems from "extremely tight hamstrings" caused by an unknown past injury to the hamstring. This influences the back somehow -- I don't understand it.
But, I would think NBA GMs will do their due dilligence and if he does fall to 21 it might be 1. because the risk is really high, or 2. because there is so much talent out there that even at low risk he isn't worth it. Will be an interesting decision if he falls.