This lamentable effort was enough to remind José Mourinho that he has far more pressing concerns than Manchester City poking fun at him in their Amazon documentary. Manchester United’s budding film critic could only grit his teeth as Brighton & Hove Albion’s supporters informed him that his city is blue. City had swaggered to another victory earlier in the day, sticking six goals past Huddersfield Town, and on this evidence United are lagging behind the league champions in the place that truly matters.

Mourinho’s pre-match comments about City’s lack off class off the pitch felt like a diversion tactic long the end of this farcical defeat. The most famous club in England never resembled a team with serious designs on winning the title, conceding three goals in a first half for the first time since Louis van Gaal presided over a collapse against Arsenal in October 2015, and a variety of talking points tested their manager’s temper. Eric Bailly and Victor Lindelöf were culpable for two of Brighton’s goals, underlining why Mourinho was aggrieved not to strengthen his central defence in the summer, and Anthony Martial’s listlessness beggared belief. It was difficult to argue with Paul Pogba’s admission that Brighton won because of their superior desire.

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Brighton relished beating United in this fixture for the second successive season. The game was played in a boisterous atmosphere and the pace was frenetic, with Chris Hughton’s players quick to the ball, strong in the challenge and brave enough to commit players forward, particularly when Anthony Knockaert and Solly March found space on the flanks.

There was nothing revolutionary about the home team’s tactics. They were efficient in their simplicity, hunting in packs as they sought to make up for last week’s meek defeat to Watford, although there was also much to admire about the move that led to Glenn Murray breaking the deadlock in the 25th minute. Knockaert had given United a warning moments earlier, bursting on to a long ball before slashing wide from an awkward angle, and it was March who provided the assist for the goal with a sharp cross from the left.

United’s lax defending must have enraged Mourinho although he spent a lot of money to bring Bailly and Lindelöf to Old Trafford. Gaetan Bong slipped a pass behind the outnumbered Ashley Young, who was entitled to wonder why Juan Mata was not protecting him, and Bailly’s decision to vacate his position in the centre gave March the room to find Murray, who stepped in front of Lindelöf before turning a clever flick past David de Gea.

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United were pulled apart with astonishing ease and Brighton pulled clear when Shane Duffy capitalised on more atrocious defending two minutes later. Perhaps the most shocking aspect of the goal was that Duffy had just gone close to forcing home Pascal Gross’s corner. The ball came back to the German and he drilled it back into the area, where it reached Duffy again thanks to a touch from the excellent Knockaert. With nobody in a red shirt minded to react, the centre-back set himself before making it 2-0.

Mourinho stood alone in his technical area, clapping furiously and urging his players to lift their heads. Seven minutes later Romelu Lukaku used his to halve the deficit, nodding Luke Shaw’s bouncing volley past Mathew Ryan after Brighton failed to clear their lines from a corner. The fans in the away end fleetingly sensed that a comeback in keeping with United’s dashing traditions was on the cards. In the event, however, little changed. United were outplayed by Dale Stephens and Davy Propper in midfield, where Fred gasped for air and Pogba showed the hunger of a man wondering what he would look like in Barcelona’s colours, and Martial’s failure to step up in the absence of the injured Alexis Sánchez was a notable disappointment.

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After seven minutes Martial spread anxiety in Brighton’s defence by running at Martin Montoya, who had a fine debut at right-back, and moments later Lukaku ought to have scored after Lewis Dunk, who would be replaced by Leon Balogun, injured himself conceding possession on the edge of the area.

Yet the first half ended with Martial receiving a yellow card for fouling Knockaert, who had hustled his fellow Frenchman off the ball, and the winger would make way for Marouane Fellaini after an hour.

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Mourinho had already replaced Mata and Andreas Pereira with Marcus Rashford and Jesse Lingard at half-time, by which point Brighton had restored their two-goal cushion. Bailly committed a rash foul on Gross, who hammered his spot-kick in off De Gea’s legs, and there was no way back for United despite Pogba’s late penalty. They had two shots on target in open play and awarding them one star would be generous.