Ole Gunnar Solskjaer's side thought it had grabbed the headlines for all the wrong reasons against Tottenham on Sunday but Jurgen Klopp's team upstaged it in a scarcely credible 7-2 defeat against Aston Villa, AFP reported.
Klopp can argue Liverpool's defeat is a bizarre one-off after an impressive start to the season but United appears to be in deep trouble.
Liverpool, which romped to the title by 18 points in the 2019/20 campaign, went into its match at an empty Villa Park with a 100 percent record in the league this season.
It was missing first-choice goalkeeper Alisson Becker through injury and was also without Sadio Mane and Thiago Alcantara, who have both tested positive for coronavirus, but was still expected to have too much for it opponent.
But Villa forward Ollie Watkins scored a first-half hat-trick and Jack Grealish had a hand in five goals as Liverpool's high line was torn apart by the home side's counterattacking.
"It was unexpected but it happened tonight," said Klopp, whose side lost just four league matches over the previous two seasons.
"We put all our rubbish things and mistakes in one game and hopefully we can start again."
The Liverpool boss said he was desperate to get his players back on the training pitch but they are now heading off for an international break before gathering again for a tough-looking challenge at table-topping Everton.
"I would love to have a training session tomorrow and Tuesday and to talk about it, but the boys go back off to international duty," Klopp said. "Hopefully they come back healthy and we use the two days to prepare for Everton."
While Liverpool will hope its result is an anomaly, there is plenty of evidence that United has lost its way.
The Old Trafford club has not competed for the Premier League title since Alex Ferguson retired in 2013 but hopes were high after it finished third last season.
That optimism has been misplaced, with United soundly beaten 3-1 at home by Crystal Palace and incredibly fortunate to escape with a 3-2 victory at Brighton last weekend.
But it sank to a new low at an echoing Old Trafford on Sunday as Harry Kane and Son Heung-min ran riot, scoring two goals apiece against the home side, which was reduced to 10 men after the first-half sending-off of Anthony Martial.
Former United defender Gary Neville, now a TV pundit, described its performance as "an absolute shambles" as it matched its 6-1 loss to Manchester City in 2011.
United's players were fortunate there were no supporters in the ground to witness the rout.
Club captain Harry Maguire apologized to fans in an Instagram post, saying the performance was "unacceptable" and Marcus Rashford said there were no excuses.
Solskjaer took the blame for the humiliation, which once again raises major questions about whether he is the man to restore the club to their former glories.
"It's a horrible feeling, the worst day I've had as a Manchester United manager and player," said the Norwegian manager.
"It's nowhere near good enough. I hold my hands up. It's my decision to pick the team I did. Also as a squad, that's not good enough for Manchester United."
While Liverpool remains favorite to win a second successive Premier League table despite its humbling, United already looks out of the running as it scrambles for answers.