Aguero charged down Lindegaard's clearance. Fortunately for the Dane, the ball ricocheted wide of the target.
Having survived that brush with a deficit, United were soon ahead. Rooney dropped between City's lines to collect the ball before spreading it wide to Valencia. The Ecuadorian swung the ball towards the far post, where Rooney bulldozed between Nani and Micah Richards to power a header against the underside of the crossbar and into the net.
The away end was presently a heaving, writhing mass of delirium - and the mood was further heightened two minutes later when City skipper Kompany was dismissed in controversial fashion. The Belgian launched into a two-footed challenge to rob Nani of possession, but cleanly won the ball. Referee Chris Foy paused before brandishing his red card, whipping the Etihad Stadium into a storm of fury.
City's initial response was one of defiance. Aguero forced a magnificent full-strength save from Lindegaard with a curling effort from the edge of the area, then smashed another effort into the side-netting after seizing on a defensive misunderstanding between Ferdinand and Jones. Soon, though, United made the extra man tell.
Nani fed Patrice Evra down the left flank and, though the Frenchman's cross was half-cleared, Welbeck pounced on the loose, looping ball and hooked an unstoppable volley into Constel Pantilimon's bottom corner from 12 yards. The game's only Mancunian starter revelled in the moment, kissing his United badge as he wheeled away in delight.
Welbeck was involved again 10 minutes later as United established a three-goal lead. The 21-year-old was needlessly scythed down by Aleksander Kolarov inside the City area, prompting the award of a clear penalty by referee Foy. Pantilimon superbly parried out Rooney's penalty, only for the United striker to calmly nod the rebound into the unguarded goal.