The NFL player's association is probably the worst out of any sport in terms of protecting its players and their interests. This article explains some of it-https://www.washingtonpost.com/news/sports/wp/2015/03/10/ndamukong-suh-max-scherzer-and-the-difference-between-lucrative-contracts-in-the-nfl-and-mlb/ but basically NFL players are almost completely at the mercy of their team. Practically anybody can (and will) get cut, and if you get cut there's a good chance your career is over.