Ole Gunnar Solskjaer said Manchester United's 6-1 defeat against Tottenham Hotspur was his worst-ever day in at the club before condemning Spurs forward Erik Lamela for his contribution to Anthony Martial's first-half sending off.
United have now lost both home games in the Premier League this season, conceding nine goals in the process, after being humiliated by Jose Mourinho's Spurs.
And United manager Solskaer, played for at Old Trafford from 1996 to 2007 and has been manager since 2018, said he has not endured a worse day during his time with the club.
"It's a horrible feeling, the worst day I've had as a Manchester United manager and player," Solskjaer said. "You don't win games of football by making individual mistakes, errors, making a performance like that.
"It's alarming, nowhere near good enough. I hold my hand up. It's my decision to pick the team and as a squad it's not good enough for Manchester United.
"When you have a defeat like this, which has happened at the club before, you've just got to look yourself in the mirror."
United were already 2-1 behind when Martial was sent off by referee Anthony Taylor on 29 minutes for slapping Lamela while defending a corner.
Lamela's reaction prompted ridicule from Solskjaer.
"This is like a very two-edged sword for me," Solskjaer said. "Anthony, well done for not going down when the lad struck you in the throat, but then you shouldn't react to it like that with a slap in his throat.
"If you do that there's always a chance they are going to send you off, but come on, if that was my son I'd put him in the attic for two weeks without food. You don't go down like that, at all. It's an absolute joke.
"If that was one of my players I would absolutely hang him up to dry because you don't go down like that."
United defender Luke Shaw suggested he and his teammates gave up following Martial's red card.
"It was a lack of concentration and then a mistake, another mistake, another mistake," he said. "We switched off. It really hurts.
"Manchester United shouldn't lose in this manner. It really hurts, it's embarrassing and we've let a lot of people down. We've let ourselves down and the staff down. We need to look in the mirror, we are nowhere near it at the moment. Maybe it's lucky that we've got a break now.
"When we went down to 10 men maybe we gave up, there wasn't that character on the pitch. We were too quiet and naive. There was many mistakes, we need to do more. "It's embarrassing. I was embarrassed on the pitch. We need a long hard look at ourselves."
Meanwhile, Mourinho showed no sympathy to his former club over the red card, adding: "I didn't watch it. Maybe later I will. If someone can cry about the VAR decisions it is Tottenham and if someone cannot cry about VAR it is Manchester United for sure.
"I don't know, the only thing I know is we played extremely well. I told my players that a good result here would be to win and we did that."