Brighton rock woeful Man Utd after Law tributes
Amorim's side were hoping to mark Law's death aged 84 by producing the kind of entertaining display that was the hallmark of the United star's teams.
Instead they crumbled to a sixth defeat in 11 league games since Amorim arrived from Sporting Lisbon to replace the sacked Erik ten Hag.
United are languishing in 13th place and although they are 10 points above the relegation zone, they are far from certain to avoid being dragged into the survival fight on the evidence of this wretched display.
Yankuba Minteh set the tone for United's moribund performance with an early opener for Brighton.
Bruno Fernandes equalised with a penalty before half-time, but second half goals from Kaoru Mitoma and Georginio Rutter left United in turmoil.
After a gritty draw at Liverpool, an FA Cup third round win with 10 men at Arsenal and a 3-1 win over lowly Southampton, United looked as if they might have turned a corner.
But, in truth, they were luck to beat Southampton and all their flaws were laid bare against a dynamic Brighton team who have won six of their last seven league meetings with United.
Amorim's side have failed to keep a clean-sheet in 12 matches in all competitions, with both their leaky defence and error-prone keeper Andre Onana exposed by Brighton.
It was a depressing result on an emotional day for United after Law passed away on Friday.
Law, diagnosed with Alzheimer's disease and vascular dementia in 2021, is the third top scorer in United's history, with 237 goals in 404 appearances.
Law was a member of United's European Cup-winning squad under Matt Busby in 1968, when they became the first English club to lift the trophy, though he was injured for the final.
Dubbed Scotland's greatest ever player by Ferguson, Law is the only player from his country to win the prestigious Ballon d'Or.
United in disarray
United fans left red scarves and flowers at the base of the 'Holy Trinity' statue outside Old Trafford that depicts Law alongside team-mates Bobby Charlton and George Best.
A piper played 'Flower of Scotland' as the teams walked out, with United legends Alex Ferguson, Brian Kidd, Paddy Crerand and Alex Stepney on the pitch as a eulogy to Law was read out over the tannoy.
There was a minute's applause and both teams wore black armbands throughout the match.
The sombre mood seemed to affect United and Brighton took the lead after just five minutes.
Carlos Baleba's long pass sent Mitoma racing clear behind the flat-footed United defence and the Japan winger squared to give Minteh with a tap-in.
United drew level in the 23rd minute when Baleba's tug on Joshua Zirkzee conceded a penalty that Fernandes converted for his eighth goal this season.
Former Manchester United forward Danny Welbeck threatened for Brighton with another goal-bound effort that was deflected to safety by Diogo Dalot.
But Brighton had to wait just a few moments to go back in front thanks to Mitoma's 60th minute strike.
With United's defence in disarray, Minteh crossed to the far post and Mitoma stretched to poke in from close-range.
Mitoma's fifth goal this season made him the all-time leading Japanese scorer in the Premier League with 15, one ahead of former Leicester forward Shinji Okazaki.
United keeper Andre Onana's frailties came back to haunt Amorim's men again in the 76th minute.
The error-prone Onana dropped a routine cross at Rutter's feet and the French forward gleefully slotted home as furious United fans headed for the exits.