Couldn't this be likened to people working with lead based paints and asbestos who later on in life won cases against employers who allowed them to work in these conditions?
I would say it's different because it's common knowledge that playing pro football has resulted in paralysis or even death in the past. So the suing players are making an argument that risking breaking their necks and being paralyzed for life was an acceptable risk, but longterm effects of concussions wouldn't have been.