Erik ten Hag's Premier League debut as Manchester United manager ended in defeat as a first half double from Pascal Gross fired Brighton to a 2-1 victory at Old Trafford.
The home side almost got off to a nightmare start when Leandro Trossard robbed Diogo Dalot of possession 20 yards from goal, although the Belgian could only lash his subsequent finish into the side netting.
United responded with a gilt-edged chance of their own in the sixth minute. The ball ricocheted across the Brighton penalty area after Scott McTominay burst into space, falling invitingly to Bruno Fernandes who ballooned his finish over the crossbar.
Christian Eriksen then worked Robert Sanchez in the Seagulls goal after United broke into space following a quick free-kick, but it was the visitors who broke the deadlock just shy of the half-hour mark.
Trossard fed a delicious pass inside Harry Maguire for Danny Welbeck to latch onto, before the former United striker pulled the ball across goal for Pascal Gross to stroke into an empty net at the far post.
Graham Potter's men then stunned Old Trafford into silence as Gross grabbed his second of the afternoon. A sweeping move from the visitors fed Solly March on the right and, when his driven effort was parried out by David de Gea, Gross reacted first to slam the rebound into the roof of the net.
Welbeck headed agonisingly wide from Trossard's goal to open the second period as the Seagulls aimed to drive home their advantage.
Marcus Rashford ought to have halved the deficit for United when he was picked out at point-blank range from substitute Cristiano Ronaldo, but the England international contrived to fire straight at Sanchez.
Eriksen worked the Seagulls stopper with a try from distance as Joel Veltman reacted first to the rebound, before United grabbed a lifeline in the 67th minute.
Sanchez flapped at the corner, with the ball bouncing off Dalot's shoulder before trickling towards the goal. Alexis Mac Allister looked to have scrambled the ball clear of danger, but, in aiming to atone for his earlier mistake, the Brighton goalkeeper knocked the ball off his teammate and across the line.
Debutant Lisandro Martinez thought he had grabbed the all-important equaliser for United as the ball fell invitingly at the back post, with only a heroic sliding intervention from Lewis Dunk protecting Brighton's advantage.
Here's how both teams rated.
Man Utd vs Brighton player ratings
1. Man Utd (4-3-3)
GK: David de Gea - 3/10 - Didn't cover himself in glory with his involvement in both Brighton goals. Ought to have cut out Welbeck's cross while he parried the ball into a dangerous area for Gross to stroke home his second.
RB: Diogo Dalot - 4/10 - Took up positions inside the pitch more commonly associated with Pep Guardiola's Man City, but his defensive positioning was wayward.
CB: Harry Maguire - 5/10 - Allowed Welbeck to run across his blindside in the lead up to Brighton's opener and looked hesitant stepping up to apply pressure.
CB: Lisandro Martinez - 4/10 - Cautioned on the stroke of half-time after being caught on the ball on the halfway line, which summed up a slightly hesitant performance.
LB: Luke Shaw - 5/10 - Caught two-vs-one at the far post for Brighton's opener. Rarely offered an attacking threat.
CM: Scott McTominay - 4/10 - Constantly looked to break beyond the space in the Brighton backline. Fortunate to stay on the pitch after a wild follow through on Caicedo in the first half.
CM: Fred - 1/10 - An afternoon to forget for the United midfielder. Caught in possession repeatedly and he was too late to react as Brighton doubled their lead in the first half. Hooked for Ronaldo.
AM: Bruno Fernandes - 6/10 - Delivered some sumptuous crosses, but he flitted in and out of the game too often.
RW: Jadon Sancho - 4/10 - Dangerous when he could break into space, but United struggled to find him in promising situations.
CF: Christian Eriksen - 5/10 - Played with his head up, seeking to stretch Brighton's defence with long passes, but he endured mixed success. Looked more comfortable when returning to midfield.
LW: Marcus Rashford - 3/10 - Struggled to get involved in the first half, although he spurned a couple of glorious chances after the interval.
SUB: Cristiano Ronaldo (52' for Fred) - 6/10 - The Portuguese arrived to a mixture of cheers and jeers, but he provided a much-needed threat running in-behind for United.
SUB: Donny van de Beek (78' for McTominay) - N/A
SUB: Anthony Elanga (89' for Sancho) - N/A
SUB: Tyrell Malacia (89' for Shaw) - N/A
SUB: Alejandro Garnacho (89' for Fernandes) - N/A
Manager: Erik ten Hag - 4/10 - His decision to deploy Eriksen as a false nine was slightly baffling and United improved following the introduction of Ronaldo in the second half. Lessons to be learned for the new United boss.
2. Brighton (3-4-2-1)
GK: Robert Sanchez - 5/10 - Produced a heroic save to deny Rashford from point-blank range, but he was solely responsible for the concession of United's goal as he flapped at the corner.
CB: Joel Veltman - 7/10 - Played under ten Hag once upon a time at Ajax and Man Utd could have certainly used his defensive assurance.
CB: Lewis Dunk - 8/10 - Marshalled Brighton's backline impeccably, dropping off when required to limit the space in-behind. Made a heroic block from Lisandro Martinez in the dying embers.
CB: Adam Webster - 6/10 - Defended aggressively, stepping up to stifle United's creative threats.
RWB: Solly March - 7/10 - Attacked with extreme aggression on the right-hand side, taking the initial shot in which Gross added his second from in the first half.
CM: Moises Caicedo - 7/10 - Took a nasty challenge from McTominay in the first half, but he shook it off to produce another impressive performance away to a top six team.
CM: Alexis Mac Allister - 6/10 - Operated in a slightly more withdrawn role to usual, but he threw himself about with enthusiasm. Unfortunate that Sanchez knocked the ball against him for United's goal.
LWB: Leandro Trossard - 8/10 - Pressed really aggressive down the left-hand side, threading a gorgeous pass through to Welbeck for Brighton's opener.
AM: Pascal Gross - 8/10 - Arrived unchecked in the penalty area to slot the Seagulls ahead, timing his run to perfection at the far post.
AM: Adam Lallana - 7/10 - Knitted play together for Brighton in his attacking midfield position, showing great ownership in possession.
ST: Danny Welbeck - 9/10 - A nuisance to the United defence all afternoon, teeing up Gross for his opener and causing problems with his runs through the heart of the defence.
SUB: Tariq Lamptey (75' for Trossard) - N/A
SUB: Enock Mwepu (75' for Lallana) - N/A
SUB: Levi Colwill (90' for March) - N/A
SUB: Denis Undav (90' for Welbeck) - N/A
Manager: Graham Potter - 8/10 - Showed personality in his team selection, using Gross in a more advanced role which paid dividends. His move to a back four in the second half almost backfired, but he was quick to correct his error.