Nottingham Forest capitalised on Manchester United's defensive blunders to record their first win at Old Trafford in almost 30 years.
Rasmus Hojlund replied to Nikola Milenkovic's early opener, but Ruben Amorim's home side were unable to get back level for a second time after they conceded twice in seven minutes at the start of the second half.
United keeper Andre Onana was at fault for Forest's second goal, shaping to make a low save to his right when Morgan Gibbs-White shot from the edge of the area, only to be beaten to his left as the ball did not move as he thought.
Onana and Matthijs de Ligt then let birthday boy Chris Wood's looping far-post header bounce between them, and Lisandro Martinez failed to react either as the ball ended up in the far corner.
United skipper Bruno Fernandes reduced the arrears after 61 minutes, but despite making five changes the hosts were unable to force an equaliser.
It means United have suffered successive Premier League defeats for the second time this season, with this setback following on from their midweek loss at Arsenal.
Amorim has won only one of his four league games in charge as his side remain 13th. Their 19 points from 15 games is United's lowest tally at this stage of a season since 1986-87. Forest, in contrast, are up to fifth.
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