Best hasn't been cleared to play yet. It's been over a year now, Burleson is advising Best to, for lack of a better term, "just give it up and save your own life". Even if Best was cleared to play at some-point this year it is an awfully large gamble to take. Just my opinion but if Best doesn't play a single game in the Pre-season he calls it a career (or at least he should).
I also don't think the Lions medical staff will ever give him the all-clear. That isn't to say some other organization won't give him the opportunity to play causing more damage. The NFL is a business, they don't care about their players. Thus the law-suit by the Players Association.