United manager Jose Mourinho blamed "incredible defensive mistakes" for the humiliating defeat on his return to Stamford Bridge, where he won three Premier League titles in two stints in charge.
"For the fourth goal, (Ander) Herrera and Paul Pogba there, I think it's absolutely scandalous," Phil Neville was quoted as saying by dailymail.co.uk.
Gary, now a Sky Sports pundit, said: "They are like mannequins in those red shirts. He (Kante) has walked through 130 million pounds of midfield. It's absolutely garbage defending."
In addition to a porous defence and trampled-over midfield, target man Zlatan Ibrahimovic looked off the pace, leaving 18-year-old Marcus Rashford to do too much work.
Having made a flying start to his United career with four goals in as many games, the 35-year old Ibrahimovic has netted just once in his last eight appearances, a 1-0 Europa League win over Ukrainians Zorya Luhansk.
Pogba, who scored twice in Thursday's 4-1 win over Turkish side Fenerbahce, is struggling for consistency in a three-man midfield crying out for a creative playmaker.
Mourinho's main concern must be the shaky defence, however, and they will face another stern test with the visit of Manchester City in the League Cup fourth round on Wednesday.
The Portuguese, who also guided Inter Milan to the treble in 2010 and won the 2012 Spanish league title with Real Madrid, apologised to United fans after the rout and dismissed any suggestion he still had a soft spot for his former club.
"The millions of fans that we have around the world, they are obviously with a very bad feeling," he told MUTV.
"I have to apologise for that as the leader of this dressing room, and the only thing I can say is that I am Man United 100 percent."
(Editing by Peter Rutherford)
