Manchester United slumped to the bottom of their Champions League group on Wednesday as 17-year-old Roony Bardghji came off the bench to score the eventual winner.
United took the lead three times in the match as Rasmus Hojlund scored twice against his boyhood club and Bruno Fernandes added a third from the penalty spot.
Marcus Rashford's red card changed the game on its head as the winger was sent off midway through the first half for a dangerous tackle when he was trying to protect the ball.
Copenhagen scored twice to level the match before two more strikes followed in the final 10-minutes as two lapses in concentration cost United a crucial European victory.