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.
It was Roberto Mancini's selection of Mario Balotelli. It was the manager's latest disregard for the law of diminishing returns. 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 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 Ballotelli 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 the forward 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 saw 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.
They might even have avoided another psychological body-blow by rescuing at least a point if Samir Nasri had not so feebly helped home Robin van Persie's winning free-kick in added time.
If Ballotelli 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.
It was also hard to avoid the symbolism implicit in the relatively quiet Van Persie's winning strike.
Earlier, the Dutchman who decided to join United rather than City had struck the woodwork and created the opportunity for Ashley 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, 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.