I don't think this is anything that the NFL will have any serious threat from. At least the NFL can argue that they have always had the best and most reasonable known standards of safety. That NFL protective gear has and continues to evolve in order to correspond to the increasing pool of knowledge. These will be hard standards to overcome regardless of how well known and how tragic the stories of NFL stars. It is like. NASCAR driver suing for getting in a car wreck.
But here's the kicker. Her is the NFL's legal trump card. Professional Boxing. This is an accepted industry. Well within societal norms. It has no evolving protections, save that displayed by the industry itself in the form of Harry Reid.