For over 10 months now, there have been major question marks over United's talisman, but Rooney answered back in the best possible way on Saturday by scoring a stunning overhead kick 12 minutes from time to give the hosts an eight-point cushion over their big-spending neighbours.
It was tough luck on City, who were impressive for long spells, and responded to Nani's 41st-minute opener by levelling through an Edin Dzeko shot which deflected in off David Silva.
But Rooney's moment of genius was worthy of winning any game, and with City surely out of the running now, maybe it will win United the Premier League title too.
Attempts from both sides to avoid inflaming what was always going to be a volatile atmosphere, even before City's startling rise up the Premier League, were breached only by a member of the Blues groundstaff, who used a social networking site to post some pretty ugly comments and also the stadium announcer, who repeatedly welcomed supporters to the "greatest football club in the world".
A goalless meeting between the sides in November had prompted complaints of negativity towards Blues boss Roberto Mancini, a feeling a similarly tepid encounter with Arsenal last month only reinforced.
Faced with a table that showed his team five points in arrears of their neighbours, having played a game more, this was the point of no return for City though - and Mancini went for it.
City should have been ahead after only three minutes.
Silva started an intricate move that also involved Micah Richards before Carlos Tevez threaded a brilliant pass through to the Spain international, leaving him completely clear deep inside the penalty area.
Edwin van der Sar advanced quickly, forcing Silva to make an instant decision, the former Valencia star turning in anguish as his delicate prod rolled just wide of the far post.
Even at that early stage, Mancini must have wondered whether another such opportunity would come.
Yaya Toure and Aleksandar Kolarov both went close as the visitors continued to press. But City could not get a clear sight of the United goal.
Not that their hosts were doing much in return.
It took them almost 35 minutes to force Hart into a meaningful save, Darren Fletcher climbing to reach Ryan Giggs' curling cross and though the Scot's header was firm enough, it lacked power.
Nani had been a peripheral figure up to that point. He did serve notice of his intentions shortly afterwards though as he fired over from the edge of the area.
Having done so well, City would have been cursing at the manner in which they were opened up.
One long ball from Van der Sar, which Rooney flicked on to Giggs. A neat pass from the Welshman and Nani was in, his first touch taking him away from Pablo Zabaleta before he steadied himself, then beat Hart with clinical ease.
City must have spent the interval cursing at the injustice of it all.
They were the ones who continued to press after the restart, although it was not until Mancini had called Shaun Wright-Phillips and Dzeko off the bench that they got their equaliser.
Even Mancini would admit it took his side a huge slice of luck to drag themselves back into it, with the substitutes combining at the start.
Dzeko's shot from Wright-Phillips' cross was going well wide of the goal when it struck Silva on the back, the deflection leaving Van der Sar totally helpless.
It looked like City would grab a winner, with Dzeko causing huge problems inside the United area.
But the inspirational figure was wearing red. For so long now, there have been questions about Rooney's form.
There can be none now, not after he met Nani's cross with a jaw-dropping overhead kick that sent the ball into the far corner, Hart rooted to the spot.