As if losing to United in a cup semi-final wasn't galling enough for Manchester City fans... that result could also help the Reds win another league title.
Sir Alex Ferguson feels the euphoria from the midweek win at Old Trafford assisted Sunday's triumph at the Emirates Stadium. And certainly the champions picked up from where they left off, taking the Arsenal game by storm with two first-half goals.
"I think Wednesday’s result and performance (against Manchester City) has galvanised us again," said Sir Alex.
"I think you could see today that the players were really ready for the game. They were sharp and good in their passing. And they made a lot of chances too.
"It was a great performance by us, there’s no doubt about that, and I was really pleased. At this time of year, you really need to kick on."
The United players reprised their previous display at the Emirates - in the 2009 Champions League semi-final - by terrorising Arsenal on the break; albeit with different personnel in Nani, not Cristiano Ronaldo, joining Ji-sung Park and Wayne Rooney in attack.
"Counter-attacking has always been part of our game anyway, particularly away from home," said Sir Alex.
"We capitalised on the opportunities today because Arsenal play a lot of good football and get to the edge of your box regularly. If you can win the ball there and counter-attack quickly, then you’ve got a good chance against them."
Although Arsenal are now four points adrift of United, and five behind leaders Chelsea, Sir Alex insists the Gunners are still very much in the title race - and he's rooting for them to win next weekend.
"I hope they go to Stamford Bridge next Sunday and batter Chelsea! That's what we need, for Chelsea to drop points obviously - and they're probably saying the same thing about us.