Grimsby's Stefan Payne was sent off for headbutting his own team-mate in Saturday's League Two match at Bradford.
Payne was seen arguing with veteran midfielder Filipe Morais as the players headed towards the tunnel at the end of the first half, with the striker then leaning in to headbutt Morais.
Grimsby, rock-bottom in the Football League, were trailing 1-0 at the time of the incident after Paudie O'Connor struck for the hosts three minutes before the break.
Referee James Bell saw the clash between the two players and quickly took out the red card, though he had to follow Payne into the tunnel in order to show it.
Grimsby's own Twitter account described the incident as "unforgivable".
Morais was then substituted in one of three Mariners changes at the interval.