The Portugal winger had already netted with a jaw-dropping leveller in what turned out to be the highest-scoring of these encounters since 1992.
He then capitalised on Vincent Kompany's blunder to win a game that seemed City's for the taking after Joleon Lescott and Edin Dzeko had taken advantage of David de Gea's hesitancy.
But Sir Alex Ferguson's introduction of three younger members of his squad at the break triggered a comeback for which United are so famed, Chris Smalling setting the ball rolling six minutes after the restart, triggering a richly-deserved victory charge for the Premier League champions.
It had been some 24 hours for Danny Welbeck.
Selected for England ahead of Jermain Defoe on Saturday, he started Sunday afternoon's showpiece whilst Dimitar Berbatov was on the bench and Michael Owen remained in his suit. The 20-year-old was part of a scintillating performance from the Red Devils that lacked only an opening goal.
Between them, Welbeck, Wayne Rooney, Nani and new signing Ashley Young cut City's midfield apart. Had United's final ball been better, they surely would have got their noses in front.
As it was, Lescott got in the way of efforts from Young, Smalling and Welbeck. Rooney curled a free-kick narrowly wide.
City's tackling grew increasingly desperate and after Mario Balotelli and Dzeko had got themselves embroiled with Nemanja Vidic and Anderson respectively, Micah Richards was fortunate to stay on the field for a studs-first lunge on Young that sent the former Aston Villa winger flying.
Referee Phil Dowd felt the need to issue some strict instructions to Blues skipper Kompany after another shuddering lunge from Nigel de Jong, this time on Welbeck.
How quickly circumstances can change.
Ferguson has already admitted to taking a gamble on De Gea, who - at just 20 - is being asked to fill the boots of a man with twice his age in Edwin van der Sar.
It had been accepted by Old Trafford's coaching staff that some work was required in the weights room for De Gea to have the best chance of dealing with the more physical nature of English football.
His lack of bulk may be the reason why he opted against coming for Silva's teasing free-kick seven minutes before the break. Neither Vidic nor Rio Ferdinand got themselves in position to make a challenge and Lescott calmly flicked it into the corner.
It got even worse for United, and De Gea, in stoppage time.
Dzeko was given far too much room in midfield to decide what to do with James Milner's square pass and after advancing 10 yards, lashed a fine shot into the United net.
It was one of those that, on first viewing, appeared to be De Gea's fault. Replays showed there had been some movement as the ball flashed in, which made life harder for the Spain Under-21 international.
However, there was a degree of culpability and clearly, De Gea is on a steep learning curve, not just in his goalkeeping but also United's DNA.
Ferguson's response was to replace three of his most senior players - Vidic, Ferdinand and Michael Carrick - at the break and introduce a trio of youngsters in Phil Jones, Jonny Evans and Tom Cleverley.
Within six minutes of the restart, Smalling had dragged the champions back into it, touching home Young's free-kick after Dzeko had completely failed to track his run.
Then came the equaliser, with Rooney, Nani and Cleverley opening City up with a devastating series of short passes before Nani beat Joe Hart with a delicate chip.
De Gea showed his raw talent by beating away a ferocious goalbound effort from Adam Johnson, then palming Richards' powerful header over.
The City duo were two of 14 Englishmen involved. The oldest member of United's second-half side was Young, at 26, during a spell following the departure of Patrice Evra and Dimitar Berbatov's introduction.
It was a taste of the future, with Cleverley in particular electrifying until he hooked an excellent chance over from eight yards.
The pity was it took a blunder to win it, Kompany failing to read Rooney's clearing punt, allowing Nani to rob him and charge forward, rounding Hart before tapping home.