What you need to know:
- It was United's second successive defeat after their 3-1 loss at Arsenal before the international break.
Troubled Manchester United plunged deeper into crisis as Brighton swept to a shock 3-1 win at Old Trafford in Saturday's Premier League clash.
Erik ten Hag's side were rocked by a first-half goal from former United forward Danny Welbeck before Pascal Gross and Joao Pedro put Brighton further ahead after the interval.
Hannibal Mejbri's first goal for the club was no consolation for United, who were booed off at the final whistle.
It was United's second successive defeat after their 3-1 loss at Arsenal before the international break.
With three defeats in five league games already this season, United are languishing in 12th place and have the look of a club in turmoil on and off the pitch.
Ten Hag appeared to have stabilised United last season when he ended the club's six-year trophy drought by winning the League Cup and qualifying for the Champions League in his first campaign.
But hopes of a title challenge are already fading after a dismal start to the new season, with a daunting Champions League opener at Bayern Munich on Wednesday threatening to add to United's woes.
In response to a rash of disciplinary issues involving his players, Ten Hag claimed this week that he arrived at Old Trafford following his move from Ajax to find "no good culture" among the squad.
On Friday, Ten Hag said he did not know whether exiled forward Jadon Sancho will play for United again.
Sancho has been banished to train away from United's first team after the England international angered Ten Hag when he claimed he was a "scapegoat" after he was dropped for the Arsenal loss.
Meanwhile, United forward Antony is on a leave of absence to fight allegations of domestic violence made against him by three women, which he strongly denies.
Antony's problems came weeks after Mason Greenwood was forced to leave United on loan to Spanish side Getafe over domestic abuse allegations that were later dropped.
United had lost their past three league games against Brighton, including last season's corresponding fixture in Ten Hag's first match in charge.
Once again, Brighton proved impossible for United to subdue, despite a bright start from the hosts.
Marcus Rashford and Rasmus Hojlund both missed early chances to put United ahead and Brighton took the lead in the 20th minute.
Adam Lallana let Simon Adingra's low cross run through his legs to the unmarked Welbeck, who side-footed home to leave Old Trafford in shell-shocked silence.
Rashford was inches away from equalising with a surging run that ended with a deflected shot that cannoned back off the crossbar.
Making his first start after his August move from Atalanta, Hojlund was controversially denied his maiden United goal when he poked in Rashford's cross, only for VAR to rule the ball had gone out of play before reaching him.
Rashford couldn't convert Hojlund's cross soon after half-time and Brighton made the most of that escape to increase their lead in the 53rd minute.
Gross has made a habit of tormenting United in the past and the German made it seven goals in 11 games against them when he collected Tariq Lamptey's pass, danced past Lisandro Martinez and fired a clinical finish into the bottom corner.
United came from two goals down to beat Forest at Old Trafford earlier this season.
But Brighton were too savvy to allow that to happen to them and Ten Hag was loudly jeered when he opted to replace Hojlund with Anthony Martial.
In the 71st minute, Pedro produced a superb finish from Lamptey's pass, prompting United fans to trudge to the exits.
Hannibal came off the bench to curl home from 20 yards two minutes later, but it was too late to stop the crisis from engulfing United.