Goalkeeper Alexander Nübel started AS Monaco's second defeat in the Europa League with a mistake.
The keeper, on loan from Bayern Munich, shot his teammate Malang Sarr (44th) in the hip when he conceded the first goal in the 0: 4 (0: 1) at Trabzonspor and thus forced the defender to score an own goal.
Manchester United's all-star side earned a victory against Cyprus outsiders Omonia Nicosia thanks to an injury-time goal.
With Cristiano Ronaldo and the additions of Antony and Casemiro in the starting eleven, the English record champions struggled to a 1-0 (0-0) and kept the chance of direct qualification for the round of 16.
The gap to leaders Real Sociedad is still three points, Scott McTominay (90 + 3) scored decisively.
Meanwhile, Monaco, who slipped to third place in Group H behind Trabzonspor (six) and Ferencvaros Budapest (nine) with another six points, are worried about progressing.
Nübel was also unable to prevent the further goals conceded by Vitor Hugo (48th), Enis Bardhi (57th) and Trezeguet (69th).