United spurned several chances to win just 2-1 yesterday.
"There was a lack of urgency in our game throughout. We were far too casual. It was an unusual Manchester United/Arsenal game. There were a lot of bookings but it never really got anywhere near the Manchester United/Arsenal games of the past.
"I was just really disappointed in our ability to hammer home the advantage," he added.
"I spoke about goal difference to the players at the start of the season. We lost [the title] on goal difference last season and I don't want to see that again. We had an opportunity to add to the goal tally today."