Manchester United manager Erik ten Hag says his side “lost our heads” in the 7-0 thrashing by Liverpool on Sunday.
The defeat was United’s joint heaviest in a competitive game while it was Liverpool’s biggest win against their rivals.
Ten Hag’s side were 1-0 down at the break before letting in six goals in the second half at Anfield.
“Second half it was not us. It was not our standards. We didn’t play as a team,” said Ten Hag.
Asked if he was angry with his side’s second-half display, he added: “Yes, definitely, and surprised because I have seen the last weeks and months a team that is resilient and determined with a winning attitude.
“We didn’t stick to the plan, we lose our heads. We didn’t do our jobs. We didn’t track back and it was really unprofessional.”
Ten Hag took over at United last summer and suffered a 4-0 defeat at Brentford and a 6-3 loss at Manchester City early in his tenure.
The Dutchman added: “We have seen in the past we can bounce back. After Brentford, after Manchester City. This is definitely a strong setback and is unacceptable.
“We let the fans down and I’m really disappointed and angry about it.
“It is a reality check… It’s a strong message [that] we get.”
The defeat at Liverpool came a week after Ten Hag won his first trophy with United when they beat Newcastle United to win the Carabao Cup.
“It was a really bad performance and the worst of the year,” said Ten Hag.
“We lost the game at the end of the first half and at the start of the second but you have to keep your head up and we didn’t.
“It’s all about showing discipline and we didn’t, so that’s when you start conceding goals.
“It’s a really unprofessional performance and unnecessary. You have to do your jobs and we didn’t.”
On the fans who travelled to Anfield, he said: “That was tough and we really have to thank them. I can’t blame them for leaving because it was a really bad performance, I’m ashamed myself for the fans today.”
The Old Trafford club remain third in the Premier League but are 14 points behind leaders Arsenal and nine adrift of Manchester City after both won on Saturday.
United’s next game will see them play Real Betis in the Europa League on Thursday (20:00 GMT) before they return to league action when they host relegation-battling Southampton on Sunday (14:00 GMT).
“We have made a lot of progress but you see what happens when you don’t keep standards,” said Ten Hag.
“What I have said in the dressing room is that this is unacceptable. But also it is only one game.
“We will bounce back. This team is strong enough and we will reset and bounce back.”
Midfielder Bruno Fernandes and forward Marcus Rashford missed promising chances for United before Liverpool took the lead and went on to dominate.
“It is very frustrating, disappointing, sad because it is a really bad result. We came here with a different mindset before the game,” said Fernandes – who was heavily criticised by Sky pundit Gary Neville after the match.
“The first half was really good from ourselves, we had a lot of chances. We controlled most of the first half I think. They didn’t create that much.
“We tried to go forward and score goals and we lost a bit of balance. We gave too much space away against this kind of team. We have to avoid that.”
Important everybody knows we’re still here – Klopp
Liverpool have got their faltering season back on track with four wins from their last five top-flight games and have moved up to fifth in the Premier League.
They are three points behind fourth-placed Tottenham having played a game less than Spurs, although Newcastle United and Brighton could move above Liverpool if they win their games in hand.
Liverpool manager Jurgen Klopp called the win against United a “special” result considering the “lower level of confidence” his team had against Ten Hag’s side after the different campaigns both teams have had this season.
“It sounds like a game from another season, it doesn’t really fit into this season so far,” said Klopp.
“It’s so incredibly important for us. We need results and for that you need performances and tonight we got both. The start of the game was super dominant and super flexible.
“We know the result is a freak but the performance was outstanding and that is what I take.
“A few months ago, everyone thought it was a good moment to play Liverpool – you can’t say it publicly but everyone thought it – because they felt we were struggling a lot but now it is less of a good moment. We look much more like ourselves.
“It is important that everybody knows we are still here and we are still alive. We have to keep going.”
Cody Gakpo opened the scoring for the Anfield side and scored two goals against United along with Darwin Nunez and Mohamed Salah, while Roberto Firmino rounded off the victory with the final goal.
“We were so clinical, so good in every area of the pitch and we couldn’t have ever envisaged this score but when players are confident, that’s what happens. Something just clicked,” said Liverpool left-back Andy Robertson.
“It felt like we were all over them and knew where the ball was going to be at all times.”