Manchester United were booed off as Charlie Austin's late winner gave Southampton a 1-0 victory at Old Trafford.
Austin came off the bench to score on his debut (87) after drawing a needless free-kick from Adnan Januzaj, the United substitute barely 30 seconds into his first appearance since returning from Borussia Dortmund.
The goal sparked a furious response from significant numbers of a home support that had been restless throughout, and the result leaves United five points off the Champions League places.
Saints' second straight win at United put paid to a five-match unbeaten run that had lifted the pressure on Louis van Gaal, who snapped at renewed press speculation over his position ahead of the game and had to endure some supporters having a very vocal say on his future as he made his way down the touchine at the end.
And the Dutchman might now have to revise for good a tentative suggestion, made after last week's win at Liverpool, that United can still play a part in the Premier League title race.
Fraser Forster did not have a save of note to make, clutching one long-range Daley Blind shot to his midriff before the break and watching another from Jesse Lingard skid wide.
And an initial upsurge in urgency after half-time, sparked by the introduction of Juan Mata and encouraged by the crowd, failed to yield anything more menacing than a couple of penalty-box skirmishes.
Instead it was Southampton, who had made and missed the only chance of the first half for Sadio Mane, who emerged as the likelier winners as they won a succession of set-pieces.
Victor Wanyama headed wide at the back post from one while Paddy McNair, on for Matteo Darmian as another United full-back succumbed to injury, denied Shane Long from another.
The decisive free-kick need never have been conceded, though. Januzaj was sent on to have an impact in the final few minutes and at least fulfilled that part of the bargain, clumsy contact with Austin giving Saints' new striker the excuse he needed to drop to his knees and claim a decision.
Even then there was no excuse for the lack of marking that allowed Austin to meet James Ward-Prowse's free-kick with the kind of sure finish that persuaded Ronald Koeman to spend £4m on him last week.
Januzaj flashed one final half-chance wide as United belatedly piled men forward, but a point would have been scarcely merited.
Charlie Nicholas's Soccer Saturday verdict
"Manchester United were really poor. It lacked speed, tempo. The passing was slow, very predictable. I didn't hear Manchester United fans vocally at all until five minutes before half-time and it was a shot which caused the anger on their faces towards the dugout. Van Gaal bought on Mata for Fellaini, a massive cheer went up. But again they never really threatened.
"He's nothing new; Charlie Austin can guarantee you goals and that's a priceless commodity. They've got him for £4m because they went and tested the water. It's probably not even a gamble but a tax write-off for some of these clubs, it's early for Charlie but he's rewarded straight away.
"This win gathers quite a bit of momentum for Southampton, all of a sudden they're starting to think is top four a possibility?"
Man Utd: De Gea (6), Darmian (6), Smalling (5), Blind (5), Borthwick-Jackson (5), Lingard (6), Fellaini (5), Schneiderlin (6), Herrera (5), Martial (4), Rooney (5)
Subs used: Mata (5), McNair (6), Januzaj (4)
Southampton: Forster (6), Cedric (6), Fonte (7), Van Dijk (6), Bertrand (7), Clasie (7), Wanyama (7), Targett (6), Tadic (5), Mane (6), Long (7)
Subs used: Romeu (6), Ward-Prose (7), Austin (8)
Man of the match: Charlie Austin