Bruno Fernandes admitted Manchester United were destined to lose the derby from the opening minutes because they did not trust themselves.
The midfielder was part of a team over-run by a vibrant Manchester City, who tore to a 6-3 victory thanks to hat-tricks from Erling Haaland and Phil Foden.
City led 4-0 at half-time and later held a 6-1 cushion, before United grabbed a pair of late replies from substitute Anthony Martial.
It was gruesome watching for anyone on the red side of Manchester, particularly as United had been on a four-game winning run in the Premier League going into the game.
"The belief from the beginning was not the best and I think that caused us many problems and made us concede early goals," said Fernandes.
Fernandes captained United in the absence of the injured Harry Maguire, but he could not imbue the side with the confidence to shake off a rocky start.
"I think the second half was much better," Fernandes said. "We kept control more, we controlled the game whenever we had the ball. We were down on the result, so we had to take some risks.
"And obviously, when we took that risk, we scored straight away. We tried to [go forward] a little bit more, but then we conceded more goals. You can't give that much space to City."
United 'won' the second half 3-2, but it was emphatically City's day.
Fernandes has just one goal and one assist in seven Premier League games this season, after 10 goals and six assists last term.
In the previous campaign he managed 18 goals and 12 assists, so new boss Erik ten Hag will be hoping to draw that mercurial level of form from the Portuguese playmaker, rather than the watered-down version that has been served up of late.
"Obviously, we are really disappointed," said Fernandes, speaking to MUTV. "But now it is time to focus on the next game and understand we have to get back the performances we did before [the international break], and the belief and togetherness, and understand that this cannot get us down. It has to make us improve for the future."
The next game for United is a Europa League meeting with Omonia in Cyprus on Thursday, while they return to Premier League duty at Everton on Sunday.
Haaland used his social media accounts to gloat after his and City's sublime performance, declaring: "Manchester is blue!"
Fernandes and United are in no position to question City's superiority at present, and it is a rebuilding job again, just as it was after the early season losses to Brighton and Hove Albion and Brentford.
United followed those setbacks by beating Liverpool, Southampton, Leicester City and Arsenal.
Everton, Newcastle United, Tottenham and Chelsea present their opposition over the next four games, with Fernandes eager to banish the derby blues.
"We knew it was going to be a tough game, but we've had tough games before and won them," he said, "so I think it's about mentality, attitude, togetherness and belief in the process."