Posted by Wally Bunker on September 25, 2017, 2:26 pm
The NFL rule book specifically requires both teams appear on the field for the playing of the anthem, standing, remaining quiet, and holding their helmets in their left hands. Failure to do so can result in fines, suspensions, and the loss of draft picks.
The rules are found on pages A62-63 of the league’s game operations manual:
The National Anthem must be played prior to every NFL game, and all players must be on the sideline for the National Anthem.