Manchester United were beaten 3-1 by Brighton at Old Trafford as the home crowd turned on manager Erik ten Hag.
Ten Hag has enjoyed the overwhelming backing of United fans since his arrival last summer and the majority have supported the Dutchman in his current dispute with winger Jadon Sancho.
But they let Ten Hag know they did not agree with his decision to replace new signing Rasmus Hojlund with Anthony Martial 19 minutes into the second period in no uncertain manner, as a cacophony of boos rang round the stadium before the Dane was applauded off.
The change didn't make much difference for United, who were behind thanks to a first-half goal from one-time home favourite Danny Welbeck, which was followed up after the interval by efforts from Pascal Gross and substitute Joao Pedro, who were both given far too much space around the United penalty box.
Hannibal Mejbri's first United goal gave the hosts some hope, but in a frenetic finish it was the visitors who came closest to finding the net again as Andre Onana made four excellent saves, the best of which came in injury-time to deny on-loan Barcelona man Ansu Fati a debut goal.
It was United's first home defeat in 13 months, when Brighton again triumphed here. The Seagulls are now on a run of four successive Premier League wins over their hosts, and this victory brought more boos at the final whistle, though not on anywhere near the same scale as when Hojlund was withdrawn.
Ten Hag's side have now conceded 10 goals in their last four games, and have also conceded more than one in four successive league matches for the first time since 1979.
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United hit unexpected trouble
Things were not supposed to be like this for United.
Six points from their opening five games represents their worst start since the 2014-15 campaign under Louis van Gaal.
In addition to the three damaging defeats they suffered, they are also without Sancho, who is training away from the first team after he said on social media he felt he was being made a scapegoat by Ten Hag in a clear challenge to the manager's authority.
Fellow winger Antony is also absent, having been given time off to address allegations made by his former partner.
With new signings Mason Mount and Sofyan Amrabat watching from the stands, Ten Hag is clearly struggling with significant membersof his squad missing.
Still, United had plenty of experience on show, but with the exception of Marcus Rashford they were largely toothless in attack.
The England forward had one first-half effort deflected onto the angle between post and crossbar, and also supplied the low cross which Hojlund, sliding in, probably should have converted.
At that point, with the game still goalless and United enjoying an encouraging start, a goal could have been pivotal.
As it was, the home side's momentum gradually petered out and there was no repeat of the heroics which saw them come from two goals down to win against Nottingham Forest last month.
Brighton on the march
While Ten Hag's side is still being viewed as a rebuild in progress, it is worth remembering the Dutchman has been in charge of his team longer than Roberto De Zerbi has been at Brighton.
In that regard, the comparisons between the two sides are stark.
Six of United's starting line-up were bought during Ten Hag's tenure, while only one De Zerbi's signing - former Borussia Dortmund midfielder Mahmoud Dahoud - started for Brighton.
Evidently, De Zerbi has made a massive impact with his attention to detail and tactical acumen, which owner Tony Bloom admitted this week have caught him by surprise.
But through Bloom, chief executive Paul Barber, skipper Lewis Dunk and beyond, there is a continuity and vision about Brighton which those in power at Old Trafford would do well to take note of.
Simon Adingra is another previously 'unknown' player making a huge impact for the Seagulls this season, as so many have done in recent years.
Bought from Danish club Nordsjaelland last year, the Ivorian was loaned out to Bloom's Belgian club Union Saint-Gilloise, before returning for the current campaign a more accomplished player.
The 21-year-old created the opener by exploiting space on the home side's left flank and delivering a cross which Adam Lallana stepped over to allow Welbeck the room to finish.
Kaoru Mitoma also travelled down the Brighton to Union SG loan route before returning to the south coast. The Japanese forward's impact is now well established and it was his pass that allowed Tariq Lamptey to set up Gross for Brighton's second, a goal made somewhat easier to convert for the German by Lisandro Martinez's fruitless dive to win possession inside his own area.
With star signin Ansu Fati making his first appearance for the club in the final moments and a European debut against AEK Athens to come on Thursday, these really are unprecedented times for Brighton.