Former Manchester United midfielder Roy Keane believes that David de Gea should have done better than allow Arsenal's second goal, and could have taken a red card to stop Danny Welbeck from scoring.
Antonio Valencia's underhit pass back to the goal was latched on to by the former United man as he slid past the goalkeeper and put the ball into the empty net for Arsenal's winner.
It was the first time that the Gunners have won at Old Trafford in almost 10 years, with their last win coming in 2006 when they completed a league double over Sir Alex Ferguson's men.
"It was shocking defending. I was critical of the first goal but that was even worse," Keane said.
"United needed to do more, they never got going in the second half. I don't begrudge Arsenal the win.
"There is a time to play and a time not to - Phil Jones should have cleared it. Valencia compounded his first half error.
"I think the keeper can take Welbeck out of the game. He would get sent off but he could have made the sacrifice," he added.
The Gunners had taken the lead through Nacho Monreal after Alex Oxlade Chamberlain latched on to a pass from Mesut Ozil and waltzed past the United backline to open up the Spaniard.
However, the euphoria was short-lived as the home side equalised moments later through Wayne Rooney after Laurent Koscielny had failed to judge a cross from Angel di Maria, setting the home fans at full voice.
However, the defining moment came in the second half with Welbeck's winner but United were kept in the game by some brilliant saves by De Gea, who atoned for his mistake in the second goal and was by far United's best player on the pitch.
Meanwhile, the draw for the FA Cup semi-finals has been revealed with Arsenal set to play Reading or Bradford at Wembley while Liverpool or Blackburn will play Aston Villa in the second semi-final.