Despite being up against a side that has cost God knows how many million to put together, the home side once again proved who was the top dog in these parts and also took a giant step towards lifting the title once again.
Yet City could have got a draw after they equalised with a goal off David Silva’s back after Nani had coolly put the home side into the lead before half time.
But it was that man Rooney who had the City fans leaving dejected once again when he scored a blinder late on in the game to give United all the points.
Mancini also paid tribute to the brilliance of the strike.
He told Sky Sports: "Only a fantastic goal can decide this game.
"A fair result was a draw. We played very well and I'm proud of my players.
"I thought we could win this game. After 1-1 I thought we had a chance to win.
"But it was a fantastic goal that Rooney scored and it changed this game."
The result leaves City eight points behind United and Mancini added: "Yes, it is very difficult (to catch United) but I think we should continue.
"Like today, when we lost a game that we don't deserve to lose, I think we can win a game that we don't deserve to win."