Make no mistake. Sir Alex Ferguson will be angry.
Was this the moment the power change in Manchester was confirmed? If City now go on to win the title, will Sir Alex hang up his boots? Is Pep Guardiola already lined up to succeed the greatest manager of all time?
Whatever happens now, City have completed a famous double over a United side that didn't have the wit, strength or guile to break them down.
This also may be the last time Sir Alex will start his two elder statesmen together. He called on his most reliable big game players and tightened up the defence to get a result which United never looked like getting.
Okay, so one error by Chris Smalling decided the match, but the simple truth was that United never looked like scoring. Giggs, Park and Scholes did nothing wrong, but Nani and Rooney could do almost nothing right.
If he stays at the helm, Sir Alex isn't suddenly going to become a pragmatist overnight. He's not going to fundamentally change his formation from 4-4-2 at home and 4-5-1 away and in big matches. But he badly needs to freshen his squad.
If he had any doubts about the need for fundamental change, those will have been dispelled on Monday. They will have been simmering since the Wigan defeat and boiling after throwing away a cast-iron win against Everton.
United don't do this. They revel in 'squeaky bum time.' They almost always come stronger as the season goes on.
This time it's different. With only one win in the last four and a pathetic four points out of 12, the title is slipping away. Short of another great performance from Newcastle United, it is City's to lose now. They have the momentum and it would be no surprise if United surrendered it at home to Swansea.
With all the scouting, research, personal visits to watch players and a dossier to choose from, Sir Alex will not hang around now. There may well be several deals already in the offing. He has three months to put together a title-winning squad for next season.
The European Championships and the Olympics are a distraction he can do without. He will want certainty as soon as possible. Then he can begin to build a relationship with his new charges.
Meanwhile, the number of players at risk will have grown. Of course many are secure for the future, but who would question Patrice Evra's departure? He has not had a good season defensively. He is one of the best attacking wing-backs in the world, but the two City defeats have shown up his woeful limitations as a defender.
It's time for wholesale change at Old Trafford in a way that we may have never seen from Sir Alex. His reputation is at risk. He knows he should have sorted the midfield problem long ago. If he still intends to continue, he has to reassert his stature in the English and European game by having a clearing out and making some marquee signings.
In this the final part of the trilogy, we examine how much room he's got–financially and manpower-wise, and whom he might like to sign into those positions.