Now, perhaps, we can understand why Pep Guardiola did not want to entertain any questions before this match about the possibility of Manchester City going unbeaten through the entire season. The Invincibles mark II? Not quite. The champions have been reminded what it feels like to lose a Premier League match, eight months since it last happened, and suddenly it is Guardiola’s team playing catch-up at the top of the table.

Those of us who have lauded them for so long would probably be well advised not to join the stampede of revisionists finding fault in what had previously looked like a perfectly oiled machine. Yet, if nothing else, at least it is a reminder that City are not as slick as many people believed. They had been behind for a grand total of 12 minutes in all their previous league fixtures and, on the evidence of this defeat, it is fair to say Liverpool, the new leaders, can be encouraged by the possibilities that have opened up.

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One bad result does not change the fact City are a supreme, brilliantly efficient and dangerous football team – quite possibly the most exciting collection of players we have seen in English football. But it was also brought into sharp focus during this display that Guardiola and his players have not, contrary to popular opinion, created a form of football perfection. Not yet, anyway.

For starters, there is no point outpassing opponents if you don’t outscore them, as was the case with City before N’Golo Kanté’s goal completely changed the course of this match. For the opening 44 minutes, Chelsea were grateful for City’s inability to turn their superiority into the hard currency of goals. Thereafter, Maurizio Sarri’s team played as though determined not to let an away side dictate play so comprehensively. Their victory was sealed by David Luiz’s 78th-minute header from a badly defended corner but, even taking into account the home side’s improvement, it must have been startling for Guardiola to see the deterioration in his own players. Their response to going a goal down was surprisingly weak and, at 2-0, there was absolutely nothing from the visitors to make it look as though they still believed the game was salvageable.

Facebook Twitter Pinterest N’Golo Kanté fires Chelsea into the lead late in the first half. Photograph: Shaun Brooks/Action Plus via Getty Images

Instead, it turned out to be the most satisfying result yet for Chelsea of the Sarri era, while Guardiola will have to accept it did not work to start the match with Raheem Sterling operating through the middle. City undoubtedly missed the injured Sergio Agüero during the parts of the game early on when they were let down by a lack of penalty-area ruthlessness. David Silva went off with a possible hamstring strain and, perhaps for the first time, City badly needed Kevin De Bruyne, the match-winner in this fixture last season.

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Tactically, Sarri did something similar to Guardiola with Chelsea’s frontline, with Eden Hazard playing through the middle and Álvaro Morata not even among their substitutes. Early on, however, it was not easy to gauge the success of this experiment, simply because the home team found it difficult to create any momentum whatsoever. Hazard could be seen turning to Sarri, arms outstretched, as if to say he was not particularly relishing his central role. The game had not reached the half-hour mark and, at that stage, not many people could have imagined Guardiola would be walking on to the pitch at full-time to congratulate the victorious players in the darker shade of blue.

Chelsea rode their luck at times but this was also a reminder that football is ultimately about putting the ball in the net rather than merely looking elegant in possession. Leroy Sané alone had three chances to open the scoring and Sterling could not get a clean contact with probably the most inviting opportunity of the opening 45 minutes. Chelsea, on the other hand, were clinical, taking the lead on the only occasion they caught their opponents out of position in the first half. Hazard slipped the decisive pass between two defenders and Kanté’s shot, from close to the penalty spot, was still rising at it flew into the roof of the net.

Until that point, the pattern of the game was a familiar one for regular City-watchers, with the away team knocking the ball around with immense confidence and streaming forward in what was, at times, a 4-1-5 formation. Ederson refrained from making any no-look passes or keepie-ups inside his own six-yard area, as he had in the midweek win at Watford, but neither was the Brazilian particularly troubled before Chelsea surprised everybody, perhaps even themselves, by taking the lead.

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Chelsea began the second half as though determined that it would not follow the same pattern as the opening 45 minutes and, having starting the match with Sterling as a false 9, Guardiola soon brought on a real one in the form of Gabriel Jesus, albeit wearing the No 33 shirt. Sané, who had lost Kane for the opening goal, was taken off, with Sterling switching to the left, and Riyad Mahrez often looking like City’s best hope on the opposite side.

By the time the game reached the hour-mark, however, City had stopped passing the ball as effectively and Bernardo Silva, in particular, had gone off the boil. Chelsea had lost Mateo Kovacic with a second-half injury but were now looking far more controlled. Kanté’s influence was growing, while César Azpilicueta’s defending typified their determination not to surrender the lead. Something strange and unexpected was taking shape: a football match in which Guardiola’s men suddenly looked devoid of ideas.

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Guardiola was aggrieved about the corner decision that led to David Luiz’s goal but the replays confirmed the ball had gone out via a faint touch off Mahrez’s foot. City’s manager ought to have been more concerned about the marking from Hazard’s corner, or the chaos that was caused by David Luiz’s long, diagonal pass in the moments before Kanté’s goal. It turns out City are fallible, after all, and the most relevant question now, perhaps, is how quickly they can shake it out of their system.