Erik ten Hag admits he has a ‘hell of a job’ on his hands after Manchester United slumped to an opening day defeat.
Skipper Harry Maguire blasted a ‘nightmare start’ as Pascal Gross struck twice to give Brighton a first win at Old Trafford in 113 years.
New boss Ten Hag pointed to costly individual errors and warned fans the team’s problems cannot be turned around quickly.
The Red Devils were booed off at the interval while there were more jeers at the end despite an improved second half.
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Ten Hag said: “It is a hell of a job.
“It’s definitely a set-back. It’s a real disappointment and we have to deal with it.
“I am not satisfied, totally, because we lose and it’s not necessary. We should have done things better.
“We gave two easy balls away and the organisation was a big mistake. We made it clear how to deal with situations. It was unnecessary.
“We have to take the lessons and learn quickly from it. We don’t have time and we have to deliver.”
Ten Hag defended his decision not to start with Cristiano Ronaldo, who declared himself ready to start on the eve of the game.
The former Ajax boss picked debutant Christian Eriksen to lead the line — with Anthony Martial injured.
United improved after Ronaldo was introduced in the 52nd minute and they pulled one back through an Alexis MacAllister own goal.
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The United boss said: “Cristiano is now maybe ten days in team training and too short for 90 minutes. You can’t force it.”
Maguire and new centre-back partner Lisandro Martinez had a difficult afternoon against former United man Danny Welbeck.
Maguire said: “It was a nightmare start to be 2-0 down at half-time.
“It’s the worst possible start. It was not good enough.