Racing did it, they regulated the max amount of fuel, they reduced the top speeds, they added safety barriers, etc...
Do it with football. Change their gear, change the allowed "hits", remove their cleats, etc... I don't actually watch football so I'm not sure what would help or wouldn't, but don't act like this is just impossible to resolve.
Yeah, people would probably hate these kinds of solutions right now, but if you get the public opinion on your side, and you start moving in that direction people will change, they will realize being able to watch a safer form of football with their favorite players in it beats being able to watch "current football" but with all their favorite players horribly injured with their lives ruined.