The Reds were outplayed for the first hour and ultimately beaten by goals scored at crucial times after Sergio Aguero and Yaya Toure gave City a commanding half-time lead, before further conversions from Aguero and Samir Nasri all-but sealed the win within five minutes of the restart.
Some pride was salvaged via Rooney's sublime free-kick in the 86th minute, a finish that capped a late improvement for United. But as he told MUTV, it was all a little too late. "It's not nice to lose to City by four goals to one. To lose by any score is bad, so we're all really disappointed."
"The way we conceded the goals, one just before half-time and two straight after, is not good enough. We know we have to improve on that, to stop giving these sloppy goals away and capitalise on some of our good play, which I thought there was bits of... it just came too late in the game.
"It was a blow (City's second) but we still thought we could get back in the game. We've been two down many times before and we came out at half-time ready to get the first goal and that would have been game on. The way in which we conceded two goals so quickly killed the game.
"We knew at 4-0 that we obviously weren't getting back into the game but we had to keep fighting and pressing, trying to make a game of it to show the fans, and ourselves, the pride we have as footballers as it's never nice to lose."