After Rasmus Hojlund cancelled out Nikola Milenkovic's second-minute opener in the first half, a pair of defensive disasters from the Red Devils allowed Morgan Gibbs-White and Chris Wood - on his 33rd birthday - to propel Nuno Espirito Santo's men into a shock two-goal lead.
Bruno Fernandes pulled one back for Man United to set up an unmissable final 30, where Amorim called on the attacking cavalry, but it was a fruitless endeavour as the 20-time English champions fell to back-to-back losses.
Following a touching pre-game tribute to Man United's much-adored receptionist Kath Phipps - who died at the age of 85 earlier this week - the Red Devils showed that they had not learned their lessons from Wednesday's corner-inflicted defeat to Arsenal.
Indeed, Forest centre-back Milenkovic exposed the hosts' set-piece failings with a ferocious header just 91 seconds after kickoff, although Man United did not take long to strike back through a rejuvenated Hojlund.
The Dane's equaliser would not breathe new life into his teammates for the second half, though, as a pair of terrible mistakes from Andre Onana and Lisandro Martinez gifted Gibbs-White and Wood two quickfire goals before the hour mark.
The latter's effort was his 25th in the Premier League for the club - overtaking Bryan Roy's previous record of 24 - although when Man United needed their captain most, Fernandes stepped up with a wonderful strike to cut the deficit in half.
However, the Portuguese's strike could not inspire a late resurgence from Man United, whose defeat could be chalked up to individual errors rather than a collectively poor team display, but the end result is the same; 13th place in the Premier League table and eight points off the top four.
Meanwhile, Forest - who have now beaten both Liverpool and Man United away from home this term - sit eight places higher in fifth, just two points below ailing Manchester City as their dream 2024-25 continues.
MAN UNITED VS. NOTTINGHAM FOREST HIGHLIGHTS
Nikola Milenkovic goal vs. Man United (2nd min, Man United 0-1 Nottingham Forest)
Set-pieces prove to be Man United's kryptonite again! After losing to two corners against Arsenal, the Red Devils fall behind to a dead-ball situation after just two minutes on home turf.
Elliot Anderson sends in a wonderful inswinging corner, and Milenkovic vs. Martinez is a complete mismatch; the Forest defender easily outmuscles the Argentine and directs a bullet header past a stranded Onana.
Rasmus Hojlund goal vs. Nottingham Forest (18th min, Man United 1-1 Nottingham Forest)
Right place, right time. Manuel Ugarte finds a gap in the Forest backline and threads a slick pass through to Alejandro Garnacho, whose shot is smothered by an onrushing Matz Sels.
However, the rebound falls at the feet of Hojlund, who cannot miss with the goal at his mercy from 10 yards out.
Morgan Gibbs-White goal vs. Man United (47th min, Man United 1-2 Nottingham Forest)
Forest repeat their two-minute trick, albeit in much more unorthodox circumstances! This one all comes about from some miscommunication between Fernandes and Leny Yoro; the former's lackadaisical attempted pass to the Frenchman is cut out by Callum Hudson-Odoi.
The ex-Chelsea man finds Morgan Gibbs-White in a central area, and the 24-year-old lets fly with a powerful 20-yard effort, which totally dupes Onana and crashes into the bottom of the net.
Gibbs-White's effort did have a bit of swerve to it, but it was more or less straight down Onana's throat; hard to argue that the Cameroonian was at fault there.
Chris Wood goal vs. Man United (54th min, Man United 1-3 Nottingham Forest)
If Gibbs-White's goal was a poor one to concede from a Man United point of view, this one is downright embarrassing.
The Forest goalscorer's cross is met by Wood, whose looping header is destined for the far corner but should be meat or drink for either Onana or Martinez covering at the back post. Instead, both men inexplicably make no attempt to intervene, and Wood's effort kisses the inside of the post before ending up in the back of the net.
A birthday goal for 33-year-old Wood makes him Forest's highest scorer in Premier League history, and he was well and truly gifted that record-breaking goal by Amorim's team.
Bruno Fernandes goal vs. Nottingham Forest (61st min, Man United 2-3 Nottingham Forest)
A captain's contribution when Man United needed it most! Fernandes starts and finishes the move, firstly playing an outstanding volleyed pass out to Amad Diallo on the right.
The Ivorian skips into the box, cuts back and repays the favour for Fernandes, who whips a phenomenal first-time strike into the far corner from inside the D.
MAN OF THE MATCH - MORGAN GIBBS-WHITE
On another day, Gibbs-White's fizzing strike would have been kept out by the goalkeeper, and Wood's header from his cross would have been cut out by an opposing player, but he heads home having both scored and assisted at Old Trafford.
Mentions also go to Fernandes and Milenkovic, but Forest's creative hub just shades this vote as the Tricky Trees further their European ambitions again.
MAN UNITED VS. NOTTINGHAM FOREST MATCH STATS
Possession: Man United 72%-28% Nottingham Forest
Shots: Man United 16-11 Nottingham Forest
Shots on target: Man United 7-3 Nottingham Forest
Corners: Man United 5-3 Nottingham Forest
Fouls: Man United 9-12 Nottingham Forest
BEST STATS
WHAT NEXT?
Amorim will soon experience another two significant Man United firsts; the Red Devils face Viktoria Plzen in Thursday's Europa League clash, his first European away game, before next Sunday's tantalising derby with Manchester City at the Etihad.
Meanwhile, a Forest side with no European commitments to attend to will be back in action in exactly a week's time, hosting Aston Villa in next Saturday's 5.30pm kickoff.
Written by
Ben Knapton