The Red Devils slumped to the foot of the Premier League for the first time in 30 years after the embarrassing 4-0 defeat at Brentford.
Even new manager Erik Ten Hag questioned his players’ hunger as he sifted through the wreckage of back-to-back losses.
As a result it is understood a planned day off for the players – who do not play again until a week on Monday at home to arch-rivals Liverpool – was cancelled on Sunday.
Goalkeeper De Gea fronted up to the TV cameras to apologise for his role in Brentford’s first two goals.
He later spoke to the club website and vowed that the players would “put our lives” on turning things around.
“It’s difficult to be honest,” said the Spaniard. “It’s still tough moments. I say this many times, but I know the lads, and everyone is working hard.
“I’m working hard, the manager’s working really hard, so that’s the beginning.
“We made a big change so these are just the first games. We need to improve a lot, listen to what the manager wants and of course start to win games.
“It doesn’t matter who we play against. We need to prepare for the game very well. We have a long week.
“It’s going to be tough of course, after this defeat. But let’s put our lives in training, put our lives on the games, and let’s play for the badge and our pride.”De Gea let Josh Dasilva’s long-range shot squirm through his hands for the opener and his ill-advised pass to Christian Eriksen gifted Mathias Jensen number two.
Ben Mee headed in the third after United failed to deal with a corner and Bryan Mbeumo hit a slick fourth from Ivan Toney’s ball, all in the first 35 minutes.
Eriksen was booed by a section of the Bees fans on his return to the Gtech Community Stadium.
The Dane enjoyed a fine spell with Brentford last season as he came back to football following his cardiac arrest at Euro 2020, but then opted to join United this summer.
Bees boss Thomas Frank said: “I understand fans, how they can react. Personally, I think we as a club should show class, respect and integrity throughout every action we do.
“Of course, there is the banter when you can sing a little bit, but booing a player? That’s not class, that’s not me. If fans want to do that, I won’t say it’s fine, but I would never do it.”