Pep Guardiola looked like he didn’t know what had hit him. As Brighton’s players and supporters wildly celebrated Matt O’Riley’s winning goal, the Manchester City manager must have been contemplating an experience he has never had in almost 20 years as a manager.

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

It was particularly sweet for the midfielder – making his Premier League debut after he was injured only nine minutes into his Brighton career in August following his move from Celtic in the summer. How O’Riley enjoyed his moment that allowed the home supporters to serenade the beleaguered City manager with chants of “You’re getting sacked in the morning” as they closed in on a memorable victory.

Guardiola had promised that his side would learn from their lapses against Sporting in midweek and he sprang a surprise by retaining teenager Jahmai Simpson-Pusey in an untried central defensive partnership alongside Josko Gvardiol. Everything appeared to be going well until João Pedro came off the bench with 25 minutes to play and, along with Hürzeler’s other substitutes, proceeded to change 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 he was proud of the way they responded to being forced to chase shadows in the first 45 minutes.

The introduction of Carlos Baleba for the second half changed the game as City retreated deeper and deeper and Guardiola must have feared the worst. He left the pitch embroiled in an exchange of words with Jan Paul van Hecke after the Brighton defender had clashed with Haaland in stoppage time as City desperately chased an equaliser.

The visitors had dominated the early proceedings as the dangerous Savinho repeatedly found space in behind Pervis Estupiñán. Bart Verbruggen spread himself wide to prevent him from opening the scoring after a burst into the box before Haaland was uncharacteristically wayward from a Phil Foden free-kick as he only succeeded in clearing the ball away from danger. But City looked far more like their usual selves and it seemed like only a matter of time until they would score.

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

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

Hürzeler said in a television interview at half-time that his side had to show more courage and 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 from Estupiñán’s cross at the end of a flowing move that was started by Mitoma.

Suddenly City looked jittery but they responded by going on the attack again. Haaland almost turned provider for Savinho if it hadn’t been for a desperate clearance from Estupiñán at the back post. Rutter’s last involvement was to head over the crossbar from another dangerous ball into City’s area as Hürzeler threw on João Pedro and Brajan Gruda and went into all-out attack mode. But the Brazilian fluffed his lines when he was played through on goal and could not force Ederson into another save.

Kevin De Bruyne was introduced for his first league appearance since mid-September as City attempted to regain control but Guardiola’s anxiety only grew with each Brighton attack. He was right to look concerned as they made an awful mess of clearing Mitoma’s cross and João Pedro pounced on the loose ball to equalise. There was no doubting 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 rescuing a point for City in nine minutes of stoppage time but there was to prove no way back as City and Guardiola were left to contemplate the unthinkable.