It's a very smart move. 24yo, degree from a strong academic institution, 650k signing bonus, any salary from endorsements, card shows, and his base salary from one year he's off to a good start in life. More would be smart to do so. Do you get to keep all of the signing bonus if you quit before the contract is over? Aren't rookie contracts 3 year deals? If you retire due to injury issues, I think you can keep it. And a concussion, even if "undiagnosed", is probably not something the NFL wants to be labeling as a non-injury at this point in time. It's not a can of worms the League wants at this point. They're not in a position to defend preemptive actions against future concussions. Honestly, if I'm 24 with all the data on head trauma and I feel I've had concussions that I would do the same.