Nilsson Lindelöf played from the start - where also Swedish Anthony Elanga got the chance, but he was replaced shortly after the half-time break.
Then it was 1-0 to Brighton at Amex Stadium.
Then narrow it down.
Three more goals within eleven minutes and a 4-0 lead for Brighton meant a new nightmare night for United, who have had a hard time getting up to level throughout the season but are still looking for a place in the Europa League.
But sixth place in the table could slip United out of their hands - and from Saturday's meeting in the Premier League, it was instead Brighton who could take their first victory at home since the Christmas holidays.
Harry Maguire could not even comfort United with a reduction goal.
It was ruled out for offside.
The loss to Brighton is a series of difficult moments that United have been through lately.
The team was knocked out of the FA Cup by the Championship team Middlesbrough, suffered a heavy loss against Liverpool at home (5-1) and lost 4-1 against Watford earlier in the season whereupon coach Ole Gunnar Solskjaer was fired.