Gary Neville says Manchester United's performance in the 1-0 defeat to 10-man Everton "smelt of complacency" and will "erode trust" in Ruben Amorim's team.
United were booed off at full-time as Everton became the first side to win at Old Trafford in the Premier League after having a player sent off thanks to Kiernan Dewsbury-Hall's first-half strike which followed Idrissa Gueye's red card for slapping team-mate Michael Keane.
Neville says United, who could have gone fourth if they won by three but remain in 10th, "were nowhere near good enough" and "an embarrassment at times" as their five-game unbeaten run came to an end.
"It's as if they've imagined in their heads they're going to be in the Champions League places, and they thought that maybe before the game," the SportNews pundit said on the Gary Neville Podcast.
"It just smelt of complacency. They weren't at it from the beginning. That is a bad one for United.
"I think they've been a lot better in the last few weeks. That probably takes us back to Brentford away.
"You can't go from the fight that they showed in certain games to that. It just erodes confidence, it erodes trust.
"We're trying to build a trust in a manager, we're trying to build a trust in a team.
"The fans booed collectively at the end. It was loud, and rightly so. That was a really poor performance.
"It's almost as if you've gone sort of like two or three steps forward, everyone's feeling a little bit better about themselves, and you've just gone back to the start again.
"You can lose football matches, but you can't lose them like that. That's nowhere near good enough, it's not acceptable."
United were without summer signings Matheus Cunha and Benjamin Sesko due to injury at Everton with Amorim forced to go with Joshua Zirkzee up front, the forward making his first start for the club since last April.
But Neville refused to accept that as an explanation, adding: "Forget [injuries to Sesko and Cunha], there are no excuses. I'm not entertaining that one bit.
"Everton dominated them with 11 men and with 10 men, in a different way of domination, but they dominated them through their fight and their spirit."
SportNews pundit Jamie Carragher says Amorim's failure to adapt his approach after Everton had a man sent off will raise serious questions about his role in the defeat.
"This is a bad night for the manager. There was an awful long time left in that game [after 10-man Everton went ahead].
"I don't understand how you can stick with something so steadfastly in a situation like that when it's basically one guy up front who's not really up front, you don't need three players there.
"It's one of those moments where I think a lot of people will really question the manager on the back of that defeat.
"They'll look at the players, and we know the players should do better, but the manager will take a lot of the blame for tonight."
Neville, speaking on co-commentary, was critical of Amorim when he brought on Diogo Dalot for the struggling Patrick Dorgu and slammed the "ambling" Luke Shaw's effort.
"You have to have urgency and make the pitch as big as possible, and you have to put as many players as possible in forward areas.
"You have Shaw, (Leny) Yoro and (Matthijs) De Ligt behind the ball. Shaw is getting forward more, but he's ambling forward, let's be clear. He's been bugging me for the last 20 minutes. He's ambling, you can't do that. He should be running forward every single time, I don't care.
"Yoro, I have a little bit more sympathy for but Shaw? That's a waste of time. I don't care. It's a waste of time. It's not conning anybody. I'm not having it.
"Ruben Amorim has a question to answer. Bringing Dalot on over there in front of Shaw? I don't quite see it. You have five at the back, why? They still don't win the first ball or the second ball. Embarrassing.
"This should be like the Alamo. Really quick, high-tempo passing side-to-side, getting into good wide areas, putting crosses in, getting bodies in attack, sustaining attacks.
"It's very slow from United. There is no presence in the box whatsoever."
