Sadly, running backs have short careers.
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.