The embarrassing defeat came after the Red Devils lifted the Carabao Cup only a week prior.
Erik ten Hag has termed Manchester United’s performance in the 7-0 thrashing by Liverpool "unprofessional and unnecessary".
Cody Gakpo, Darwin Nunez, and Mohamed Salah each bagged a brace, while the seventh goal was netted by Roberto Firmino.
An unimpressed Ten Hag said the performance was below United’s standards and that his players failed to stick to the plan.
"Second half it was not us. It was not our standards. We didn't play as a team. Yes, definitely, and surprised because I have seen over 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. It was a really bad performance and the worst of the year.
"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," the Dutchman said, as per BBC Sports.
However, the former Ajax manager is confident United will rise from the crushing defeat.
After Ten Hag took over, a 4-0 defeat by Brentford and a 6-3 reverse at Manchester City represent the heaviest defeats he has suffered at United.
"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," he continued.
The defeat at Anfield is United's first 7-0 loss since they were thrashed by Wolves in December 1931. They were also beaten by the same score by Blackburn Rovers and Aston Villa in 1926 and 1930, respectively.