Of all the calamities descending upon Manchester City at the end of their week in something resembling football hell, there is no doubt about the one that could most easily have been avoided.
If Mancini is to survive a project of vast investment but apparently endemic flaws, he must surely jettison his wild but dogged belief that Balotelli, for all his extremely randomly applied talent, has a place in a serious football team capable of consistent achievement.
That City slipped six points behind United in the title race, and lost two years of impregnability at the Etihad Stadium, had so much to do with the latest bone-headed contribution from the young player who seems incapable of operating anywhere but his own weirdly constructed world.
When Balotelli performed a most languid, indeed utterly disinterested, back-heel at a time when his embattled team might have expected at least half-a-dozen more professional options, Mancini's patience finally snapped.
He yanked Balotelli off and sent on Carlos Tevez. He gave himself an authentically committed and balanced team.
He had a player who, whatever his questionable past, was plainly interested in making a significant contribution. Tevez linked with other team-mates, notably his superbly motivated compatriot Sergio Aguero, and for most of the second half City ceased to look as though they were masquerading as champions of England.
If Balotelli exposed one huge shortfall in competitive character, Nasri revealed another. No wonder magnificently fired-up Pablo Zabaleta embodied the concept of despair when the ball sneaked just inside Joe Hart's far post.
Zabaleta had been a prime mover in City's recovery. Now he had been required to witness a team-mate's surrender.
When Nasri half-heartedly raised his right boot to create a killing deflection for United's winning goal, it was a football version of putting your helmet on a stick and poking it above the line of the trench.
It was behaviour inappropriate in a hugely rewarded member of a team expected to conduct themselves as champions.
Nasri invited the strongest reprimand from the manager of a monumentally expensive team which this season has given so much dispiriting evidence that in the most pressing circumstances it simply cannot generate the right levels of nerve and application.
Balotelli, after so much previous evidence that he is not ever likely to understand the essential demands of a team sport, invited only one plausible reaction.
It was for Mancini, having seen his protege once again march down the tunnel apparently unmoved by the possibility of a devastating defeat for his team-mates, to deliver a final instruction, one which said simply: "Keep walking."
This was the worst possible conclusion to City's week of accumulating nightmare.
The need to root out the destructive element in the stands that produced the missile-thrower who landed a direct hit on Rio Ferdinand is one workaday chore in an English game which continues to be afflicted by the most rancid spirit.
But for Mancini there has to be, in its own way, an equally pressing demand to provide some convincing explanation for his decision to go in with the ultimately unreliable Balotelli when Tevez and Edin Dzeko were available.
By comparison, the old warrior Alex Ferguson, still nursing deep wounds after that 6-1 undressing by City last season, came up with a near perfect game plan.
He attacked the essentially narrow formation of City with the bite of Antonio Valencia and the elusiveness of Ashley Young and provoked, most vitally, from Wayne Rooney a performance of superb acumen and calm.
His two goals were a reminder that in all the ebb and flow of his extraordinary career, there is always the underpinning of exceptional football intelligence.
It was also hard to avoid the symbolism implicit in the relatively quiet Van Persie's winning strike, despite the assistance he received from Nasri's inept reaction in the defensive wall.
Earlier, the Dutchman who decided to join United rather than City had struck the woodwork and created the opportunity for Young to score a goal that was unjustly disallowed.
The real significance, though, was in the fact that United's new main man, the one who was bought to deliver still another title, remained a threat to the last breath of the most important domestic game thus far in a season marked by dwindling professional standards.
Van Persie, as he was last season as captain of Arsenal, is a notable exception.
He is a game-winner of proven quality and admirable consistency. He was, even Mancini might concede, everything that Mario Balotelli was not.
It is a fundamental and, you have to suspect, a decisive difference. (© Independent News Service)