Pep Guardiola looked like he didn’t know what hit him. As Brighton players and supporters wildly celebrated Matt O’Riley’s winning goal, the Manchester City manager must have been reflecting on an experience he has never had in almost two decades as a manager.

A fourth defeat in a row, for the first time in his career, will have been particularly difficult to swallow given how much his side had dominated Brighton in the first half. But a stunning turnaround from Fabian Hürzeler’s side was sparked by João Pedro, who equalized Erling Haaland’s opener before setting up fellow substitute O’Riley five minutes later.

It was particularly sweet for the midfielder, who made his Premier League debut after picking up an injury at Brighton just nine minutes after joining from Celtic in August. How O’Riley relished the moment when the home fans were able to serenade the beleaguered City manager with chants of ‘You’re getting fired in the morning’ as they closed in on a momentous victory.

Guardiola had promised his side would learn from their mistakes against Sporting in midweek and he pulled off a surprise by keeping teenager Jahmai Simpson-Pusey in an untested central defensive partnership alongside Josko Gvardiol. Everything seemed to be going well until João Pedro came off the bench with 25 minutes to play and, along with Hürzeler’s other substitutes, changed the game.

The Brighton manager had described his first meeting with the reigning Premier League champions as an opportunity for his side to challenge the establishment and was proud of the way they responded when forced to chase shadows in the opening 45 minutes .

The introduction of Carlos Baleba in the second half changed the game. City retreated deeper and deeper and Guardiola must have feared the worst. He left the field involved in an altercation with Jan Paul van Hecke after the Brighton defender collided with Haaland in stoppage time as City desperately hunted for an equalizer.

The visitors had dominated the early proceedings as the dangerous Savinho repeatedly found space behind Pervis Estupiñán. Bart Verbruggen spread wide to avoid opening the scoring after a burst into the penalty area, before Haaland was uncharacteristically wayward after a Phil Foden free-kick as he only managed to clear the ball away from danger. But City looked much more like their usual selves and it seemed only a matter of time before they scored.

The goal came promptly after 23 minutes when a lovely shimmy from Mateo Kovacic gave him space in midfield to play the perfect through ball for Haaland, who smashed the ball home from close range after Verbruggen had made the first save. The Brighton goalkeeper was fortunate that the Norwegian’s next effort, after being set up by Foden, came off his heel and rebounded off the post before Haaland headed over the resulting corner.

Brighton’s only goal was a snap from Kaoru Mitoma that went harmlessly wide and they had no answer as City had more than 70% possession. Kovacic was next to force Verbruggen into action from distance, but the hosts could well have been awarded a penalty when Gvardiol’s hand blocked Danny Welbeck’s shot on the ground after Walker misjudged a bouncing ball. The Brighton striker was not happy and could only direct a free-kick just wide of Ederson’s post after Rico Lewis was warned for bringing him down.

Hürzeler said in a television interview at half-time that his team had to show more courage and that they came flying out of the blocks. Mitoma could only direct a soft header on target from Georgino Rutter’s cross before Jack Hinshelwood should have done better after an Estupiñán cross at the end of a flowing move started by Mitoma.

Suddenly City looked nervous, but they responded by going on the attack again. Haaland almost became Savinho’s supplier, if Estupiñán had not made a desperate move at the back post. Rutter’s last involvement was to head another dangerous ball over the crossbar in City’s area as Hürzeler threw at João Pedro and Brajan Gruda and went into all-out attack mode. But the Brazilian messed up his lines when he was played through on goal and was unable to force Ederson into another save.

Kevin De Bruyne was introduced for his first appearance since mid-September as City tried to regain control, but Guardiola’s fears only grew with each attack on Brighton. He was right to look concerned when they made a terrible mess of clearing Mitoma’s cross and João Pedro pounced on the loose ball to equalize. There was no doubt about the quality of the winning goal when a one-two between Welbeck and João Pedro allowed O’Riley to steal the headlines.

Gvardiol came closest to saving a point for City in nine minutes of stoppage time, but there appeared to be no way back as City and Guardiola were left to contemplate the unthinkable.