Actually I meant to write 'opt in' but I think you got it.
Sure, they knew the protocols, but they had a reasonable expectation of them being effective. If it turns out they are not (and we aren't there yet) then all bets are off. You're talking about people looking at (in theory) losing millions of dollars of potential future earnings if they get COVID and have a bad recovery. If things get out of control (they aren't now), it'd be a no brainer to sit out instead of putting that at risk.
The union would not need to be involved in a suit. Players got enough money to hire their own layers, ambulance chasers work on commission. Protocols are in place but they could assert that NFL was negligent in their specification, their implementation, their enforcement.
At some point the risk of such would make NFL really consider if they are making a mistake continuing the season.
Be real, the flip side of that is thinking that a story featuring three unnamed NFL players and five staffers from one team catching a life threatening virus two days after playing in a game would get little traction.
The NFL is lucky this is a small market and not particularly successful team. Could you imagine if this was one of the best teams, or a major market team, or (god forbid) our team? Would you really expect people to say "well, there's contingencies in place, let's move on"?