Joe Schilling quit from his PFL light-heavyweight fight against Donegi Abena following a headbutt in the opening round.
Abena attempted to headbutt Schilling 37 seconds into the contest in Brussels as they were grappling on the canvas.
The strike missed as Schilling, fighting for the first time in almost seven years, reacted sharply to move out of the way.
Suriname’s Abena, making his MMA debut, was subsequently deducted a point by referee Mike Beltran.
The referee tried to restart the fight from the position at which it had been paused – with the American on his back – but the 42-year-old protested then verbally tapped.