His was a freak accident, made possible only because of his great strength. Normal ligaments were not made for the strength of his legs, and he pulled his thigh right out of its socket. He could never recover from that. I think he would have had at least 10 years as a running back, maybe 12, and 20 as a baseball player. The one wild card for a running back is knee injury. That can happen to anyone, and if it does, then yes, the career is shortened.
Ligament strength is why baseball pitchers will rarely be able to throw any harder, and why there have been so many blown out elbows. The current trend is to have pitchers throw as hard as they can, and the elbow ligaments just can’t handle the repeated stress. Until someone figures out a way to strengthen ligaments, some athletic activities will never see much more improvement.