The NBA has a rule, it’s not hard to do. You just require that players who appear in uniform to play a league game must stand at respectful attention for the flag and anthem.
The NBA has avoided this problem for 20 years, since it first arose for them around 1996 (there was a Muslim convert who was refusing to stand with his team for the anthem), The NBA forced him to stand, although they did say he could hold his hands in prayer to show he was praying for his own beliefs.
If the NFL really has to weasel around they could allow players whose “conscience” bothers them to stay in the locker room until just before the game begins. But under no circumstances should they allow players to utilize the uniform, stadium, team, league, and TV CONTRACTs to stage their personal “protest.”
Also, the NFL has no problem with tightly regulating what commercial and uniform logos, images, and slogans may appear for their games. They restrict players VERY tightly for their own sponsorship and commercial reasons. You can’t wear a non-approved piece of apparel, etc. Goodell has no problem doing it for other reasons — he could end this problem today if he were not such a liberal weasel.
Baseball seems to have put the race card back in the deck years ago, there are all types of players today and no racial problems.