Former club captain Gary Neville said the performance had been an “absolute disgrace, a shambles”.
He added: “It’s not their usual spirit and form. [United manager Erik] Ten Hag will deal with it, I’m sure, like he’s dealt with other difficult issues.”
It is the worst result of Ten Hag’s managerial career, analytics company Stats Perform said.
Liverpool had eight shots on target, missing only once.
Another former United skipper, Roy Keane, said the senior players “didn’t show any leadership skills” and the team as a whole would be “ashamed of their performance”.
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“Thank God I’ve never been part of a team that’s been beaten by that much playing for Manchester United,” he said.
Liverpool, who’ve had a disappointing season given their recent trophy-winning success, went in at half time just 1-0 up, courtesy of a classy strike from Dutch winger Cody Gakpo.
Then, in the second half, there were a further six goals for the Reds. Gakpo took a second, while Darwin Nunez also claimed two, as did Mohamed Salah.
Roberto Firmino completed the rout.
It means Salah has become Liverpool’s all-time top scorer in the Premier League, with 129 goals in six seasons.
“It’s very special, I can’t lie,” he told Sky Sports.
“This record was in my mind since I came here. After my first season I was always chasing the record.
“To get it today against United with that result is unbelievable. I’m going home to celebrate with the family. Chamomile tea and sleep.”
The previous record holder was Robbie Fowler, with 128.
United midfielder Bruno Fernandes said the defeat was “frustrating” and a “really bad result”.
He added: “We gave too much space away, we gave too much time. It’s just about now going to the next game and getting the result back.”
The 7-0 win – alongside losses this weekend for rivals Newcastle and Tottenham – mean Liverpool are in a good position to make an assault on the top four, and therefore qualification for the Champions League.
They are three points behind fourth-placed Tottenham with a game in hand.
For United, there were also 7-0 losses to Aston Villa in December 1930 and Blackburn Rovers in April 1926, Stats Perform said.