It is on the most exalted stages that you can feel the loneliest. This, at least, was the experience of Pep Guardiola, who in his 15th season as a manager, endured his most harrowing Bernabeu ordeal.
The Real Madrid ultras did not just settle for gloating at his misfortune, as Manchester City found themselves blown off course by the jet heels of Kylian Mbappe. They openly trolled him, screaming: "Guardiola stay!" Once, he might have been tempted to respond, with a three-fingered salute to denote his three Champions League titles, perhaps. But this time he just prowled the touchline pensively, his head bowed and his mind scrambled.
Of all City's horrors this season, none can have hurt Guardiola as grievously as this limp, tame surrender to opponents whom he once specialised in humiliating. As recently as 2020, the Catalan was known here at the Bernabeu as the Antichrist, achieving a record sixth victory at the home of the club he had grown up despising.
Despite four subsequent attempts, he has still not added a seventh. What wounded him most this time was that his players never looked like doing so, flunking his instruction to play the "perfect game" so spectacularly that by the second half, Nico Gonzalez's only solution to his failings on and off the ball was to resort to tackling rugby-style.
For somebody who has made it his signature to dismember teams with controlled, fluid, patient precision, this was torture for Guardiola. City were bystanders at their own wake, with Josko Gvardiol turned so effortlessly inside-out by Mbappe that he had to watch the Frenchman's first goal while still on his backside.
Mbappe was finding it all so easy that his celebrations consisted of insouciant hand gestures to illustrate how he had dissected City's defence on his own. In the end, those meant to be marking him retreated into full "after you, Kylian" mode, with Phil Foden standing so far off him en route to the third strike that he might as well have ushered him past.
By the end, Guardiola had seen enough, registering not a flicker of emotion as Gonzalez snaffled a meaningless consolation goal in injury time. So wretchedly has City's campaign imploded, with the club facing a struggle even to qualify for this tournament next season, you half wonder if he might contemplate a sabbatical.
He has done it before, of course: in 2012, he spent a year in New York to refresh, forming a friendship with Garry Kasparov and learning German to prepare for his next assignment, at Bayern Munich. While these are starkly different circumstances, Guardiola looks in desperate need of some rejuvenating force. Nobody doubts the intensity is still there, but the joy has long since evaporated.
You want to remember the Guardiola who ransacked Real, who with Barcelona masterminded a 6-2 filleting of the hated enemy at this stadium in 2009, before crushing them 5-0 at the Nou Camp the following year. He has savoured similar sensations at City, with the 4-0 demolition of Real at the Etihad surely the finest performance by an English club in Europe in the past 20 years.
It is staggering to reflect that this happened a mere 21 months ago. That treble-winning side - built around a midfield of Kevin De Bruyne, Ilkay Gundogan and Bernardo Silva - looked built to last. An era of true European pre-eminence, you sensed, was being forged.
Those foundations have not so much crumbled as liquefied. Gundogan has proved the old rule that, in football as in life, you should never go back. His return to City after a spell at Barcelona has been the feeblest encore, with this formerly reliable midfield metronome outmanoeuvred so frequently by Real that it was sad to witness.
Bernardo also appeared older than his 30 years, as Real made a mockery of his efforts to wrest control by passing the ball out from the back. Guardiola has been admirably loyal to his hardy perennials. That faith, though, is starting to seem like a miscalculation.
The worry is that he is also struggling to galvanise City's latest acquisitions, the players he has entrusted with leading a turnaround. A line-up heavily reliant, in Erling Haaland's absence, on Gonzalez, Omar Marmoush and Abdukodir Khusanov never threatened to trouble players of the calibre of Vinicius Junior, Rodrygo and Mbappe.
The ease with which Vinicius skipped beyond Khusanov verged on embarrassing. And, in the end, such mismatches added up to a brutal truth: that despite spending £180 million on an emergency revamp, Guardiola has failed, for the first time in his career, to lead a team into the last 16 of the Champions League.
When his name was read out before the final whistle, Real's supporters responded with a chorus of piercing derision. It was, in its way, a curious tribute, a testament to the fact that he had orchestrated their downfall more often than any other manager. What cut the deepest was that "Guardiola stay!" refrain at the final whistle, taunting him with the suggestion that Real at last had the measure of him.
There is little sign that the balance of power will be corrected any time soon. These are two clubs on contrasting trajectories, with Real re-energised by Mbappe's arrival and City at a loss to know how to replace their ageing stars with fresh blood.
What this all means for Guardiola's reputation is difficult to gauge. It was only last September, after all, that one of his Champions League adversaries was likening him to Gaudi. To many, he will forever be the transcendent genius bending the game to his will.
It is too early to claim his moment has passed. But one certainty is that this result, coupled with the unforgivable meekness of the performance, will haunt him.