Pep Guardiola marched across the turf at the end with head bowed and a face like thunder, barely making eye contact with his players or daring to glance up at the anguished away support behind the far goal. This was everything he had feared it might become, a chastening evening when hope flared only fleetingly and, in truth, his team had merited their mauling. Monaco are one of the more thrilling young sides in Europe but City, semi-finalists last year, and their manager, had expected far better than this.

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It was the feeble nature of their first-half showing which left Guardiola seething, a display utterly lacking in snap and bite, cohesion and even conviction. Elimination on away goals gives the impression this tie was tight, and there was anxiety for the home side to endure in the last quarter for all that City effectively held the ascendancy for only six minutes. But the memory that will linger of these two matches will not be that rat-a-tat of goals in the last 20 minutes at the Etihad but the manner in which the visitors were overwhelmed in the opening period of the return. That is what shocked Guardiola to his core. He is not used to seeing his own outclassed, even bullied.

How the advantage from the first leg was frittered and City’s safety net surrendered, with the Catalan’s pre-match instruction still fresh in his players’ minds, was truly disturbing. The manager rejected the notion that selecting only one midfielder charged with shielding a fragile back-line – and Fernandinho’s instincts are forever forward – might be construed as risky against the most scintillating attacking team in any of Europe’s elite five leagues.

His assertion that his team had “forgotten” their capabilities from the outset, until reminded of them again at the interval, did not reflect well on the management, but he was in no mood to accept deep-lying defensive deficiencies had undermined this European campaign. At least John Stones, dismissing that first-half showing as “very sloppy”and “not up to standard”, was more accurate with his pay-off to the television cameras: “We just weren’t solid enough.”

Three of this back five are out of contract in the summer and there is scope there for huge improvement. This merely underlined as much. For long periods there had been no semblance of defensive organisation, with Stones offered minimal protection by Leroy Sané and Aleksandar Kolarov wilting at his side. Monaco were still magnificent, hassling and harrying, snapping at City whenever the visitors attempted to catch their breath. They swarmed forward in waves spreading panic throughout, with Fernandinho overrun and Kevin De Bruyne’s distribution awry in the frenzy.

If Kylian Mbappé or Thomas Lemar was shrugged off, there was Benjamin Mendy, Valère Germain or Bernardo Silva pouncing to close down space, pilfer possession or provoke the error. The absence of Radamel Falcao, nursing a bruised thigh, mattered not. “We didn’t let them touch the ball,” said Leonardo Jardim. “They couldn’t play anywhere on the pitch.”

Neither could they escape. Mbappé was irresistible, a player destined for greatness allying pace, strength and such confidence in front of goal. The 18-year-old forced Willy Caballero into an early save after Fernandinho, attempting to build patiently from the back, had been dispossessed by Germain. City did not recover their composure at the resultant corner, first allowing Mendy to waltz away from the dithering Sané and Sergio Agüero and, when Stones blocked the full-back’s cross, permitting Bernando Silva to fizz over a centre anyway. Mbappé loitered unnoticed at the near post to turn in the cross through Caballero. It was his 11th goal in as many games and his team’s 124th of the season.

It was never likely to prove their last of the night. The ease with which Lemar and Mendy exploited the space beyond Bacary Sagna was damning and Stones, abandoned by Sané, was unable to stifle their threat alone. The centre-back found himself pulled out of position yet again just before the half hour, with Mendy eventually liberated to the byline to deliver. Fabinho, a man-mountain alongside Tiemoué Bakayoko in the middle, opened up his body to connect sweetly with a side-foot and the momentum had switched.

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At least City responded, though Guardiola’s post-match frustration reflected the fact he had expected this urgency from the start. There was a flurry of opportunities after the interval with Monaco suddenly becalmed, as if braced for their opponents’ revival.

Agüero was denied by Andrea Raggi’s intervention and then skyed an attempt over the bar. Sané, more recognisable as an attacking force, had created the latter and it was the German who offered the visitors their brief hope, scoring with the rebound after Danijel Subasic pushed away Raheem Sterling’s low shot.

Yet City’s glee was a deception. De Bruyne, enduring such a torrid night, conceded a free-kick from which Lemar’s delivery looped over Kolarov in the centre. In eased Bakayoko to plant the tie’s decisive goal into the corner of Caballero’s net, leaving Leicester alone to carry England’s hopes in this competition and City plunged into introspection.