Until Michael Olise’s stunning free-kick shocked Erik ten Hag’s side, the trip to the Emirates Stadium felt like a title-decider.
United were on course to go second in the League and move six points behind the leaders and a win at the Emirates would have seen them close the gap to three.
Still, despite bossing much of the game at Selhurst Park, Olise stepped up to leave United in disbelief.
The night had already been marred by Casemiro’s late booking, picked up for a challenge on Wilfried Zaha during the dying embers of the game.
That yellow card took the Brazilian, who has a profound impact on the team since a £70m move from Real Madrid in the summer, up to five and duly ruled him out.
Speaking to Sky Sports, De Gea said: “Big blow to lose Casemiro for Sunday. I don’t get it why Arsenal don’t play as we have to play in the same week and they don’t.
“Now we are missing one of our best players. I don’t understand. Now we miss him for a big game and it is a big loss for us."