In the 57th minute, Pascal Gross slotted in the third goal. This was a match gross in name and in nature for Manchester United and the scoring did not finish there.
While the stadium was subjected to an inordinate wait for the fourth goal, the mutinous United supporters chanted "You're not fit to wear the shirt". None of the players will be suing.
United's players heard that at Old Trafford against Norwich three weeks ago. It was a watershed moment then and the mutiny is no more militant than among the away-dayers.
Read more: United player ratings vs Brighton
Some rallied, vowing "We'll keep the red flag flying high, 'cause Man United will never die". But United have flatlined and are in urgent need of the open heart surgery Ralf Rangnick diagnosed.
The United fans started chanting Ole Gunnar Solskjaer's name, seemingly out of solidarity that he had to manage the shower in front of them. At full-time, Cristiano Ronaldo applauded them and David de Gea apologetically raised his hands. "You're not fit to wear the shirt," they chanted.
"How s--t must you be? We're winning at home," crowed the Seagulls. Brighton had prevailed once in the league at the Amex all season before United laid down their weapons.
Liverpool (twice), Manchester City, Leicester, Brighton and Watford have all put four past United this season, split evenly between Rangnick and Solskjaer. Michael Carrick should have perhaps extended his 11-day honeymoon as caretaker.
This was a Biblical defeat in an apocalyptic season for United, reminiscent of the Easter Sunday surrender at Everton three years ago. This season has been their worst in decades and It is most certainly the worst United team in a lifetime. They have shipped 56 league goals, the most in a top-flight campaign since 1978-79. Dave Sexton's soldiers still battled to an FA Cup final and the Red Army had Jimmy Greenhoff at Hillsborough.
It typified this anodyne side that Edinson Cavani, whose mind has barely been in it as much as his body has been in the country, was the most urgent player to restart after Marc Cucurella thundered in the second on 49 minutes. United were a zombified throng as they ambled back into position and there were only 49 minutes on the clock. There might as well have been 90.
United wanted it to end. Brighton didn't. There was anguish on their fans' faces as they narrowly missed a fifth goal from Danny Welbeck's attempt as they threatened to inflict a record defeat on United in the Premier League era. "We want five," chanted the crowd.
The squad disharmony from United was all too palpable. Alex Telles had to be instructed where to position himself by Rangnick, Bruno Fernandes and Ronaldo. Fernandes, prone to berating any teammate, made an exception for a ropey Ronaldo.
Darren Fletcher and the goalkeeping coach Richard Hartis pointlessly appealed for a handball against Leandro Trossard. That was not as pathetic as them instructing one of the players to appeal to the referee. The Video Assistant Referee was merciless. That made it 4-0 after an hour.
United won three times at the Amex during the behind-closed-doors era, scoring three goals. The last time they played with a crowd present, they shipped three under Jose Mourinho. This time it was more.
Again, United showed their true colours in a season where the adrenaline rush that matchgoers bring has rumbled almost every regular squad member. Finishing second in a sterile atmosphere in front of sparse stands was misleading.
Moises Caicedo may never come close to the pantheon of the ones-that-got-away from United but in a season where anything that can go wrong will go wrong, he was fated to score against them.
United stepped away from negotiations for Caicedo last year as they had become 'messy'. Caicedo, an Ecuador international prior to his move to Sussex and a defensive midfielder, might have plugged the gaps in a porous midfield he and Yves Bissouma bossed. Either would enhance the United side.
Caicedo only made his first Premier League start at Arsenal last month and has stayed in the Brighton XI ever since, helping them to wins at the Emirates, Tottenham, Wolves and now a shellacking of United. He crowned his introduction to Premier League football with a measured goal.
Defensive midfield has been the glaring issue for United, now contemplating a £60million investment in Kalvin Phillips. Caicedo, a £5m recruit by Brighton 15 months ago, has quickly claimed four top-eight scalps. Scott McTominay's inferiority complex was so glaring he committed four fouls inside the first 50 minutes.
How apt Caicedo's precise strike was sparked by hesitancy from McTominay, a midfielder United cannot programme into a defensive mindset. McTominay and Victor Lindelof left a loose ball for each other, Telles's headed clearance was hopeless and Caicedo had oodles to size up a shot that rolled past a delayed David de Gea.
This latest dire day for United included the heckling of Anthony Elanga with 'You're just a s--t Jesse Lingard'. Elanga did not make it beyond half-time but was replaced by Cavani rather than Lingard. Peculiarly, a minority of United fans chanted the seldom seen Cavani's name.
As full-time loomed, United fans chanted 'Viva Ronaldo', almost as if to absolve him of their chant the others are not fit to wear the shirt. They're not.
