Glenn Murray marked his 200th Brighton appearance with the opening goal in the 25th minute, before further goals from Shane Duffy and Pascal Gross — from the penalty spot — left United reeling. Romelu Lukaku scored for United to make it 2-1 and captain Paul Pogba converted a stoppage-time penalty with virtually the last kick of the game. By then, a battling Brighton side had done enough to earn a second straight home victory over Jose Mourinho's side.
A surly Mourinho spent the buildup to this clash choosing to criticise rivals Manchester City's fly-on-the-wall documentary released this week, saying "you can't buy class" in reaction to the behind-the-scenes account of last season at the Etihad Stadium.
When Brighton's fans sang "Your city is blue" in the final moments of the game, humiliation for United and Mourinho was complete. This was just the 10th time in Premier League history that United conceded three first-half goals, and the first time since their 3-0 defeat at Arsenal in October 2015. It punctured any optimism created by the 2-1 win over Leicester last week.
Brighton toppled United 1-0 on the south coast in May en route to a 15th-place finish in last season's Premier League and was at it again three months down the line. Murray conjured a tidy finish to put the hosts ahead, the 34-year-old striker flicking home a finish at the near post.
Two minutes later, Duffy slotted in from close range after United failed to clear a corner. Lukaku made no mistake with his second chance, after fluffing an early opportunity, but just when United was expected to force its way back into the game, more poor defending cost the visitors again.
Bailly upended Gross from behind to concede a penalty that the Austrian converted past David De Gea. Mourinho used the interval to remove Juan Mata and Andreas Pereira, with England internationals Marcus Rashford and Jesse Lingard entering as United sought an injection of pace.
Mourinho then threw on Marouane Fellaini in place of Anthony Martial on the hour, but still United continued in listless fashion. Pogba converted his second penalty in as many games after Duffy upended Fellaini, but it was too little, too late for United.
Mourinho will now demand an instant reaction when United hosts Tottenham in its next game.