He was thrust into what could easily prove the endgame in his effort to make Manchester City a significant factor in Europe when Ajax, the fledgling team fashioned by the old warrior Frank de Boer on a fraction of his adversary's resources, erupted with a strike that ripped away a reality that had been forcefully, if briefly imposed.
What followed made the dramas at Old Trafford, Arsenal's pratfall at the Emirates and reigning champions Chelsea's defeat in Ukraine, seem like merely passing discomforts.
City had stumbled into the eye of a storm substantially of their own creation after Samir Nasri, exploiting perfectly some exceptional approach work by Micah Richards and James Milner, had given them the lead with exquisite timing and placement.
It was a beautiful goal that disabled them, the most haunting reminder of those days when Ajax made great players on behalf of their own glory rather than the predatory instincts of a legion of wealthier clubs, but it was also a huge reproach to the richest of today's fat cats.
And not least it was to their frequently care-worn manager Mancini.
The trouble, it seemed at that dramatic moment just on half-time in the Amsterdam Arena, is that the City manager who is such a lion at home – at least in domestic competition – seems for ever doomed to be a pussycat on the wider stage of the European game. That had to be the interim conclusion when City, again fighting for their Champions League life in the first half of the group stage, conceded the lead – and maybe their first moments of genuine authority in their attempt to establish themselves as a significant European force.
For Mancini his existence is now brutally simple. He is involved in a fight not just for survival but some minimal belief that he will ever get the hang of what is demanded in the upper levels of European football.
When the Ajax captain and most prolific scorer Siem de Jong brilliantly drove home a cross from the highly promising Ricardo van Rhijn it was as if Mancini's entire life in the world's greatest club competition had just passed before his eyes.
There was another such devastating sensation 12 minutes into the second half when defender Niklas Moisander, one of the few Ajax players who looked passably equipped to physically compete with City at set pieces, put his team into the lead with virtually a free header at a corner.
This was, after all, a City team which not only desperately need to win but had apparently already cuffed the brightly skilled but essentially toothless Ajax into submission. City had nothing to lose but their chains, yet soon enough they were entangled.
It meant that Mancini's expression could hardly have been more pained or eloquent. He was announcing his own awareness that he was a man on the brink, one who had declared that victory in the Netherlands was vital for European life and, maybe just as pressingly, Mancini's own as manager of vast resources but sharply dwindling aura.
He was in even deeper crisis when Ajax's nearest thing to a superstar, the young Dane of infinite promise but not always hard-edged achievement, Christian Eriksen, scored a third goal with fine timing and wonderful patience.
So what did he do? He did, once again, all that was left to him. He sent on Mario Balotelli and Carlos Tevez to join Edin Dzeko, he lumped on some extremely expensive talent up front and it was, of course, in lieu of a coherent game plan – or any sense that this is a team who can consistently impose its strength.
Catastrophically, Yaya Touré – who has so often rescued City from their failed ambition and even will –was scarcely a factor and long before the end it was apparent that the intelligence and the bite of Sergio Aguero were being swallowed up in a progressively hopeless cause.
On the touchline the despair on Mancini's face inevitably deepened. Young, fragile Ajax – pointless in Europe up to last night – not only survived, they made City look a parody of the champions of the richest football league in the world.
Having been beaten in Madrid, outplayed by Borussia Dortmund at home, City were scarcely for purpose last night. For Mancini there was just one last disaster. It was the declaration of the defender Richards that the team had been bewildered by the defensive convulsion which came with a switch from four to three at the back. The players, he said, were unfamiliar with "the system". At least, Ajax attacked Mancini's fading credibility from the front.
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