Mourinho's side were blown away on the south-coast as Brighton scored twice in two minute.
PIC: Man Utd boss Jose Mourinho addresses his players during their match against Brighton. (AFP)
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Brighton 3 Man Utd 2
Jose Mourinho admitted Manchester United had paid the price for "incredible mistakes" in their dismal 3-2 defeat at Brighton on Sunday.
Mourinho's side were blown away on the south-coast as Brighton scored twice in two minutes thanks to goals from Glenn Murray and Shane Duffy midway through the first half.
Romelu Lukaku got one back, but Pascal Gross's penalty capitalised on more poor United defending to send Brighton into the interval with a 3-1 lead.
United rarely looked like mounting a fightback and Paul Pogba's penalty with virtually the last kick of the game came way too late.
A shell-shocked Mourinho conceded United's players had only themselves to blame for their first defeat of the Premier League season.
"We were punished by the mistakes we made. For me that's the story of the game. We made incredible mistakes in some crucial moments. Mistakes that killed us," Mourinho said.
"Especially that third goal -- we expected to come at 2-1 with positivity -- then the third goal killed us off in the second half. Our second goal came too late.
"The week leading up was good and I expected the team collectively to be better. There are basic mistakes that makes almost an impossible mission to win the match."
Many of Mourinho's stars, including World Cup winner Pogba, new signing Fred and French forward Anthony Martial, were anonymous for long periods, but the boss refused to criticise individuals.
"I don't want to go in that direction. I have to learn from my reality," Mourinho said.
"Last season when I was critical of my team and was debating openly of the performances I was criticised for being too open about my players.
"I will be open when they are doing well. I cannot criticise the desire of my players."
France midfielder Pogba took blame for his lethargic display, admitting he had been well below his best.
"I'm disappointed, with my performance and the team performance. We didn't deserve to win," Pogba said.
"I will always try. I know I lost a lot of balls which shouldn't happen.
"I tried, I kept pushing, that's my personality. I tried to help the team as much as possible. It didn't happen today."
Questioning United's desire, Pogba said Brighton had wanted the points more.
"Maybe we didn't have the attitude to break them, to kill them when we had to kill them and to go through the lines. That's a lesson we have to keep in mind," Pogba said.
"Today the truth is we didn't play well enough. They had more hunger than us and the result is the right one for them."