The Red Devils boss said that his team failed to capitalise on a number of chances and should have won by a greater margin.
"I was disappointed to be honest with you, [it was a] game where we should have won by four or five," said the Scot. "I think I need to sleep on it, I could not believe the chances we missed.
"We should have put it to bed a long, long time ago and you look at that scoreline 2-1 and you think it was a close game, but it wasn't a close game.
"I think there was a lack of urgency in our game throughout, far too casual. It was an unusual Manchester United-Arsenal game really."