The woe at Manchester United deepened as Brighton surged to a comprehensive 3-1 win at Old Trafford on Saturday afternoon.
Danny Welbeck struck first despite a positive early start from United before Pascal Gross doubled the visitors' advantage with a sweet finish at the start of the second half.
Substitute Joao Pedro made it a 3-0 advantage as United were carved open far too easily in the 71st minute before Hannibal Mejbri reduced the home side's deficit with a fierce drive from distance.
It's a second league defeat in a row for United and their third already this season, leaving them in the bottom half of the table, while Brighton sit behind only Manchester City, Tottenham and Liverpool at the division's summit.
How the game unfolded
United's availability woes meant Bruno Fernandes was deployed behind two central strikers in Marcus Rashford and Rasmus Hojlund in a 4-3-1-2 system, while there was also a first start for Sergio Reguilon at left-back. Lisandro Martinez was deemed fit enough to partner Victor Lindelof in central defence after suffering an injury scare in the defeat at Arsenal.
Evan Ferguson was only fit enough for the bench for Brighton as Welbeck took the striker role. Neither Solly March or Pervis Estupinan made the matchday squad, with Tariq Lamptey coming in at right-back for his first league start since March.
The Seagulls' XI contained six changes and their usual fluency was absent as United started the first half in threatening fashion.
The recalled Jason Steele kicked away a Marcus Rashford effort which lacked power after a mazy run before the England forward nearly found Hojlund with a dangerous cross. The Danish striker was inches away from connecting with a desperate dive.
Brighton settled into a more steady rhythm but the lead they grabbed just before the halfway point of the first half still came against the run of play.
Simon Adingra's cross from the right was missed by Lindelof - accompanied with a Lallana dummy - and Welbeck arrived in the centre of the penalty box to finish past Andre Onana.
That goal took the zip and vigour out of United's attacking play but they nearly lucked into an equaliser with Rashford again driving forward. His strike from distance ricocheted off Joel Veltman, off the outside of the stanchion and away to safety.
Hojlund wheeled away celebrating what looked like a United equaliser shortly before half-time, but his tidy finish was chalked off by VAR as the ball drifted past the byline before Rashford crossed to his strike partner.
United had enough momentum and quality of chances to force themselves level but instead more dithering defending from the Red Devils helped Brighton double their lead.
Kaoru Mitoma drove forward on the left and found Lamptey, whose pass dribbled across the box and dropped to Gross. The newly-capped Germany international sold Martinez with a dummy before firing in his seventh league goal against United, having struck twice in this corresponding fixture last season.
Anthony Martial and Hannibal Mejbri were introduced from the bench as Erik ten Hag searched in vain for a way back into the game but the team's sloppy shape became more disorganised as the game wore on.
Brighton's third was all too easy from a United perspective as Lamptey ran unopposed before cutting it back to Pedro, who opened his body to find the far corner.
Hannibal's fine strike from distance did offer United a brief moment of optimism with more than 20 minutes remaining but no further threat was forthcoming as Brighton followed up their victory over Newcastle before the international break with another win at Old Trafford.
Manchester United player ratings (4-3-1-2)
GK: Andre Onana - 5/10 - Given little chance of keeping out Welbeck's strike. Mainly a passenger as the ball hit the back of his net twice more.
RB: Diogo Dalot - 6/10 - Never far away from the action against Mitoma but held up well overall.
CB: Victor Lindelof - 4/10 - Couldn't get anything on Adingra's cross before Welbeck finished. Looked lost a lot of the time.
CB: Lisandro Martinez - 4/10 - Given the slip by Gross as United went 2-0 down. Looked far from his sharpest.
LB: Sergio Reguilon - 5/10 - Looked to make some meaty challenges early on during his United debut. Caught out of position fairly regularly.
CM: Scott McTominay - 3/10 - Offered little box-to-box threat from midfield or any protection when Brighton's attackers came into central areas. An overly cautious selection from Ten Hag.
CM: Casemiro - 4/10 - A little sluggish with Brighton's midfielders quicker to the loose ball.
CM: Christian Eriksen - 4/10 - Part of the static defending that gave Gross room to make it 2-0. Final ball wasn't there.
AM: Bruno Fernandes - 4/10 - Looked capable of making the defence-splitting contribution he has so often before but it never came.
ST: Rasmus Hojlund - 6/10 - Thought he'd equalised with a tidy finish but VAR adjudged the ball to have gone out.
ST: Marcus Rashford - 6/10 - United's biggest threat in the first half, hitting the woodwork with a deflected effort and nearly setting up Hojlund for an equaliser.
Hannibal Mejbri (65' for Casemiro) - 7/10 - Had a crack from distance to get United on the scoresheet.
Anthony Martial (65' for Hojlund) - 4/10 - His introduction from the bench so early on shows the limited options at Ten Hag's disposal.
Aaron Wan-Bissaka (85' for Reguilon) - N/A
Alejandro Garnacho (85' for Martinez) - N/A
Facundo Pellistri (85' for McTominay) - N/A
Subs not used: Daniel Gore, Harry Maguire, Jonny Evans, Altay Bayindir (GK)
Erik ten Hag - 4/10 - Trialled out a new system with a few wingers unavailable. Chance creation wasn't an issue but United were carved open time and time again, a problem Ten Hag will need to address before the Champions League returns next week.
Brighton player ratings (4-2-3-1)
GK: Jason Steele - 6/10 - Made an important save from Rashford early on and showed neat feet while playing out from the back.
RB: Joel Veltman - 7/10 - A couple of crunching tackles and was rarely beaten out wide.
CB: Jan Paul van Hecke - 6/10 - Looked particularly anxious when one-on-one with Rashford.
CB: Lewis Dunk - 7/10 - Some trademark moments of composure in the box.
LB: Tariq Lamptey - 8/10 - Booked early on for a late challenge on Dalot. Picked out Gross and Pedro for two assists on his return to the starting XI.
CM: Pascal Gross - 8/10 - Started the move with a sweet diagonal pass before collecting Lamptey's cross to grab another goal against United.
CM: Mahmoud Dahoud - 7/10 - Composed performance alongside Gross in midfield, even if he was dribbled past a few times
RW: Simon Adingra - 8/10 - Grabbed an assist on his first Premier League start and was an electric presence out wide.
AM: Adam Lallana - 7/10 - Buzzed around and linked play neatly, helping Brighton's midfield out-think United's.
LW: Kaoru Mitoma - 7/10 - Enjoyed his battle with Dalot and looked more dangerous late on. Surprisingly not overly involved in any of the three Brighton goals.
ST: Danny Welbeck 7/10 - Given a free run to connect with Adingra's cross for the opener.
Ansu Fati (64' for Lallana) - 6/10 - Brought a decent save from Onana after coming on for his debut.
Joao Pedro (64' for Welbeck) - 7/10 - Given plenty of space but netted Brighton's third to take the game away from United.
Billy Gilmour (77' for Dahoud) - 6/10
James Milner (77' for Lamptey) - 6/10
Evan Ferguson (85' for Adingra) - N/A
Subs not used: Carlos Baleba, Adam Webster, Igor, Bart Verbruggen (GK)
Roberto De Zerbi - 9/10 - Slow start was more than compensated for with a brilliant performance after escaping that early pressure. De Zerbi had the likes of Ferguson and Fati to call on from the bench, maintaining Brighton's attacking threat even as the clock ticked towards full time.