Tottenham's troubled campaign also hit new depths on Sunday with a 3-2 loss at relegation-fighting Everton, but Nottingham Forest's unlikely title challenge goes from strength to strength after a 3-2 win against last-placed Southampton.
At Old Trafford, the wonderful striker Law, who died this week aged 84, was the subject of emotional tributes, led by former boss Alex Ferguson, before the match.
Manchester United lining up to pay tribute to the memory of the late, great Denis Law. (AP PHOTO)
But United then couldn't conjure up any of Law's old competitive spirit as goals from Brighton's Yankuba Minteh, Kaoru Mitoma and Georginio Rutter consigned Ruben Amorim's men to a 10th defeat in the league.
Bruno Fernandes scored for United from the penalty spot but couldn't prevent a seventh defeat in 15 games overall under new coach Amorim.
United fans were taunted by Brighton supporters as many made their way to the exit before the final whistle as Amorim admitted his team may have hit a new low in the club's storied history.
"We've got all the bad records as losing games at home, losing games in the Premier League, so we are really underperforming. And we need to win games, that's all," he said.
United are now 13th in the table after their defeat, with Spurs two places below them in the 20-strong league, a perilous position for a club which had expected to be involved in another push for Champions League qualification.
Instead, it's Forest shooting for the stars after a fine win over the Saints - that's nine games unbeaten now in all competitions - featured goals from Elliot Anderson and Callum Hudson-Odoi, before Chris Wood nodded in his 14th of the season.
Forest are now third, behind Arsenal only on goal difference, with Liverpool still leading the way six points ahead of their pursuers with a game in hand.
Everton's Dominic Calvert-Lewin celebrates his goal in the win over Spurs. (AP PHOTO)
Everton embarrassed injury-hit Spurs with their first Premier League goals since Boxing Day in a one-sided opening 45 minutes, leading 3-0 at the break which is something they hadn't achieved at Goodison for almost eight years.
Dominic Calvert-Lewin, with his first in 17 matches, and Iliman Ndiaye both scored before the unfortunate Archie Gray turned the ball into his own net for the third.
Dejan Kulusevki scored with 13 minutes remaining and Richarlison grabbed another back in stoppage time but the late fightback couldn't disguise the comprehensive nature of Spurs' defeat with Australian boss Ange Postecoglou increasingly under pressure.
With the League Cup semi-final second leg against Liverpool - Spurs lead 1-0 - not until February 6 at Anfield, speculation over Postecoglou's future is mounting.
Elsewhere, in a return to some EPL normality, Phil Foden scored twice and Erling Haaland celebrated his bumper new deal with a goal as Manchester City romped to their biggest Premier League win in more than two years, with a 6-0 thumping of relegation-threatened Ipswich.
Haaland helped himself to a goal at the end of a week in which he committed himself to the club for the best part of a decade, while Mateo Kovacic, Jeremy Doku and James McAtee were also on target as City moved above Newcastle into fourth place on goal difference.
It was their biggest league win since hammering Forest by the same scoreline in August 2022 and means they're back in the top four for the first time since December 1.
With agencies
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