Here is a great article about NFL players' reactions to the whole thing:http://www.sportsline.com/nfl/story/7073223
I seem to recall NBA players saying similar stuff to what the NFLers are saying, about how 18-year-olds aren't physically, mentally or emotionally ready for the pros and so on. This time, I think it's right. Football is uniquely demanding, at least physically, and I'm afraid that if Clarett ever even makes it, he'll get broken in half.
Joe Namath once said of the NFL, "It ain't a league of angels." Beyond the usual bloodlust, there will be guys out there to kill any teenager who thinks they can cut it.
Clarett is not the phenom that it will take. IMO, the only guy who would have been close is Vick, and he's injury-prone anyway.