Enzo Maresca celebrated emphatically while Iliman Ndiaye raised both arms toward the visiting supporters.
Enzo Fernandez had won the set-piece opportunity and lay on the turf with a satisfied expression.
Fifteen minutes remained in the contest yet Manchester United appeared confused, bewildered and ultimately vanquished.
Shortly afterward, Fernandez moved into the penalty area, evaded Harry Maguire and collapsed under contact.
The United captain reacted furiously, shoving the £125million acquisition to the ground.
Both men received yellow cards.
That incident proved Fernandez’s final contribution as Nico O’Reilly entered the pitch to take his place.
As Fernandez completed his lap of honour, home supporters voiced their displeasure while the travelling City contingent offered sustained acclamation.
A fresh protagonist had emerged.
Fernandez had been outstanding after the interval as City delivered a commanding display despite being reduced to ten men for the majority of an away fixture against their fiercest rivals.
Phil Foden had committed an ill-advised challenge resulting in his dismissal before the 25th minute.
United struck the woodwork twice around halftime, presenting City with the sternest examination of their season to that point.
Maresca’s side endured the pressure, gathered themselves and ultimately secured a meritorious three points.
Gianluigi Donnarumma remained untroubled until stoppage time when he produced an excellent save to deny Bryan Mbeumo.
Earlier, Fernandez had been on his feet urging on his colleagues from the substitutes’ bench as a wayward United cross drifted harmlessly out of play.
United demonstrated no clear purpose, creativity or inventiveness after falling behind.
While Michael Carrick’s outfit must examine their own deficiencies, this represented another significant achievement for Maresca.
His choice to withdraw Rayan Cherki at the interval, despite the Frenchman having performed adequately, and introduce Ndiaye proved extraordinarily profitable.
The replacement offered willing support and impressed with his ability to retain possession.
Removing Fernandez, notwithstanding his considerable influence after the restart, also proved judicious as O’Reilly entered to protect the lead.
When the concluding whistle sounded, Maresca sprinted along the technical area toward the elated away following.
The group celebrated passionately on the Old Trafford surface with Fernandez prominent among them.
Fresh connections are forming and fresh protagonists are appearing, even if Erling Haaland continues finding the net and deciding matches.
Maresca’s side have begun their campaign in dreamlike fashion.
The citizens of Manchester’s blue sector enjoyed restful slumber that evening.