“Can we play you every week,” sang the delirious Brighton fans whose team administered a loss that has Manchester United veering close to crisis territory.

This was a third United defeat from five Premier League outings – greeted with jeers by the home support – and the second on the bounce, leaving Erik ten Hag’s side with six points from 15. Lose at Burnley next Saturday and they will be engulfed in a plight that was not in the early season script.

Brighton, rightly, ended the game cock-a-hoop. Danny Welbeck, Pascal Gross and João Pedro the scorers for the visitors who were threatened only sporadically apart from the opening 15 minutes from United.

Two-nil down on the hour, Ten Hag acted, introducing Hannibal Mejbri and Anthony Martial for Casemiro and Rasmus Højlund. The latter move was greeted with boos for the manager – a sign of the times – whose desperation was marked by Mejbri having not featured for two seasons.

Casemiro looks despondent after Brighton’s second goal at Old Trafford

Casemiro (right) and Lisandro Martínez react after Brighton double their lead at Old Trafford. Photograph: Oli Scarff/AFP/Getty Images

On 71 minutes it got worse: one of Roberto De Zerbi’s changes was João Pedro and, as with Brighton’s previous two goals, the striker was left unmarked and able to blaze past André Onana, who should have saved.

This prompted some fans to depart but Mejbri, perhaps disgusted at his colleagues’ capitulation, strode forward and powered a piledriver past Jason Steele. However this was United’s brightest moment on an afternoon that started bathed in optimism.

More to follow.