Grealish and Mahrez come off the bench to spark Manchester City win at Chelsea

It was a night when Pep Guardiola twisted on the tactical front and then twisted some more. For much of a slow-burning encounter, part of the battle was to decode the schemes of the Manchester City manager.

His most recognised left-back, João Cancelo? Pinned high on the right wing. The returning Kyle Walker? On the right of a defensive three. The midfield was flexible to the point of indecipherable. And for the opening 45 minutes, it did not work.

So Guardiola twisted again, swapping Cancelo and Walker for Rico Lewis and Manuel Akanji at the start of the second half. City remained similarly set, broadly a 3-2-4-1 in possession, and they began to squeeze higher, to find their pass-and-move grooves. Then Guardiola twisted yet again. This time it was decisive.

There were 60 minutes on the clock when he introduced Riyad Mahrez on the right and Jack Grealish on the left. It was a double substitution to highlight the riches at his disposal and one that surely turned Graham Potter a shade of green. The Chelsea manager had lost Raheem Sterling and Christian Pulisic to injuries in the early running, on the back of seeing Mason Mount ruled out with a training-ground knock. Potter can now count his senior absentees on all of his fingers and thumbs.

Within three minutes Grealish teased over a low cross, Mahrez swooped to finish and City were on their way to cutting Arsenal’s lead at the top of the table to five points. It was a lovely team move, out to the right, then back over to the left, with Kevin De Bruyne prominent. Grealish’s assist was a beauty and the only question was why Kepa Arrizabalaga did not stick out a hand before it reached Mahrez. The ball had seemed to be there for the Chelsea goalkeeper.

It was a big win for City – particularly after the frustration of their 1-1 home draw with Everton on New Year’s Eve – and it had been in doubt as Chelsea pushed in the first half. Yet it never seemed to be in the second half and certainly not after the goal.

Potter had brought on Pierre-Emerick Aubameyang when Sterling limped off in the second minute but he would substitute the substitute as he chased the game. Hakim Ziyech was also hooked. Potter sent on the 19-year-old Omari Hutchinson for his debut on the right wing and the 18-year-old Lewis Hall at left-back, sticking with his 4-2-3-1 formation and simply rotating the personnel.

The equaliser never looked on and Potter was left to reflect on another blow to Chelsea’s hopes of a top-four finish. The club have attacked the January transfer market, announcing the capture of another centre-half – Benoit Badiashile from Monaco for £33.6m – before the game. They also want to add the midfielder Enzo Fernández from Benfica and the winger Mykhailo Mudryk from Shakhtar Donetsk.

Chelsea lose Christian Pulisic with an injury midway through the first half. Photograph: Tony Obrien/Reuters

It feels as though there are moving parts everywhere, a relatively new ownership group in a hurry to splash the cash and drive change quickly. But on the field, Potter’s team have taken just six points from an available 24. The table does not make for pretty reading. They are not only the fifth-placed team from the capital but the third from west London, behind Fulham and Brentford. Next up for them in the FA Cup on Sunday is an away game against City, who have already dumped them out of the Carabao Cup.

Pulisic was hurt when the excellent John Stones stretched into a saving block challenge on him in the 17th minute. The winger was in after a nice Kai Havertz pass after a mix-up between Cancelo and Walker, and it was a bright first-half showing from Chelsea.

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Mateo Kovacic and Denis Zakaria caught the eye in midfield, while Havertz drifted into dangerous spaces. City needed a Bernardo Silva tackle to snuff out Pulisic’s replacement, Carney Chukwuemeka, after a Havertz pull-back. And it was Chukwuemeka who so nearly broke the deadlock just before half-time. After another Havertz pass, he cut inside from the right and watched his shot beat Ederson, via a slight deflection, and rebound off a post.

Guardiola had started with Phil Foden pinned high on the left and it seemed as though Rodri was the only vaguely fixed point in midfield. Then again, he sometimes dropped into the defensive line. It was strange to see City so disjointed in the first half. And yet when Erling Haaland is around, no defender can ever relax. Out of very little on 38 minutes he crackled to life, taking an Ilkay Gündogan pass with a fabulous first touch, veering away from Kalidou Koulibaly and unloading a shot in what felt like the same pre-programmed movement. The back-lift was minimal to give Koulibaly less chance to get across, Arrizabalaga less time to react. It whistled narrowly over.

Guardiola was not happy at the interval. But his mood improved as Lewis and Akanji settled, the former tucking in alongside Rodri and also covering at right-back out of possession. Bernardo Silva went wide right. Nathan Aké headed against the crossbar from a De Bruyne cross after a short corner routine while Bernardo Silva beat the unconvincing Marc Cucurella to pull back for De Bruyne who blasted off target.

Thiago Silva had lashed wide at the other end but it felt as though a City breakthrough was coming. They were enjoying more time on the ball and were getting in around the sides. Enter Mahrez and Grealish. Chelsea could not respond and Haaland almost added a second, narrowly failing to reach a low De Bruyne cross.