- Hannibal Mejbri (73')
- Danny Welbeck (20')
- Pascal Groß (53')
- João Pedro (71')
Despite making a bright start at Old Trafford, United fell behind in the 20th minute when Danny Welbeck slotted home against his former club when left unmarked in the penalty area.
United responded well and thought they had levelled just before the break through big-money signing Rasmus Højlund, on his full debut, but the ball was adjudged to be out of play when Marcus Rashford pulled the ball back to the Dane.
Substitute Hannibal Mejbri did get one back for the hosts with 18 minutes left but Brighton held on with ease to move up to third in the standings on 12 points from five matches. United dropped to 12th on six points.
After another tumultuous week in Manchester, with Brazilian forward Antony facing assault allegations and England winger Jadon Sancho's issues with coach Erik ten Hag still playing out, the club needed matters on the pitch to lift their supporters.
With Højlund looking lively and Rashford taking the attack to the visitors they looked primed to get back to winning ways, but defensive frailties proved their undoing as Welbeck's fourth goal against his boyhood club put them on the back foot again.
Rashford continued to look the most likely to level in the opening period, striking the angle of post and crossbar following a trademark driving run.
Last season's top scorer went close just after the break, but clinical finishing from Gross took him to four goals against United at Old Trafford, with only Steven Gerrard and Mohamed Salah scoring more there as a visiting player.
There were boos all around Old Trafford as Hojlund was replaced by Anthony Martial, but it was a Brighton change that put the game to bed, with record signing Pedro arrowing a fizzing effort into the net.
Hannibal's first goal for the club did not threaten Brighton's superiority, with the visitors having further chances to extend their lead as they strolled to victory.
- Jarred Gillett
|Brighton & Hove Albion||15||7||4||4||+5||25|
|West Ham United||14||6||3||5||0||21|