After conceding twice from corners against Arsenal on Wednesday, it was immediately clear that United hadn’t learnt their lesson here as they trailed to Forest inside two minutes for the second consecutive home H2H.
Elliot Anderson’s delivery was powered home by the relatively unmarked Nikola Milenković, with his first goal for the Tricky Trees, and the seventh United have conceded from corners in the PL this term - a joint-league high.
It also marked the division-best 12th time Forest have opened the scoring this season, but the hosts were far from dismayed, dominating proceedings before drawing level on 18 minutes.
A slick move went from back to front as Manuel Ugarte slotted in Alejandro Garnacho, whose shot could only be parried by Matz Sels into the path of Rasmus Højlund, who turned home his third goal in as many appearances.
Building on that equaliser was tricky for the Red Devils, who were being undone by their inability to defend set-pieces. Jota Silva headed against the bar from a deep free-kick, while Murillo slotted wide from a corner as the visitors rallied.
However, Nuno Espírito Santo’s men were next to be indebted to the woodwork as Bruno Fernandes’ venomous free-kick rattled the crossbar, ensuring honours were even at the break.
With the contest delicately poised, Forest were the ones to take the initiative, striking twice within nine minutes of the restart thanks to a defensive collapse from the hosts.
First, Morgan Gibbs-White’s low swerving effort seemed to completely wrongfoot André Onana, who made a mess of trying to save an effort that nestled in the middle of his goal.
The visitors’ third was arguably even stranger, as a combination of Onana, Lisandro Martínez and Matthijs de Ligt failed to deal with Chris Wood’s looping header, which settled in the far corner to make the striker Forest’s all-time leading scorer in the PL.
Old Trafford, and indeed Amorim's frustration was evident, but Fernandes stepped up to rifle an effort into the far corner to reduce United’s deficit with half an hour still to play.
A raft of substitutions and stoppages disrupted the rhythm of the contest though, allowing Forest to take the sting out of United’s attacks, which were infrequent to say the least.
A fizzing late volley from Amad Diallo was blocked by Morato as the visitors secured just a second win in 15 H2Hs, while marking the first season in which they’ve secured league victories at both Anfield and Old Trafford.
United’s seven-match unbeaten home run therefore comes to an end, leaving Amorim’s men in the bottom-half of the table and with plenty of work still to do.