Manchester United manager Jose Mourinho said his players made "incredible defensive mistakes" as he was beaten 4-0 on a humiliating return to Stamford Bridge.
United conceded within 30 seconds as Daley Blind, Chris Smalling and David De Gea all failed to stop Pedro.
Gary Cahill took advantage of poor defending at a corner to make it 2-0 before the break, Eden Hazard added the third just after the hour and N'Golo Kante skipped through for the fourth.
Mourinho told Sky Sports: "In the first minute, you analyse the goal and you see what happened. We made incredible defensive mistakes, individual defensive mistakes, and we paid for that.
"It started bad. After a few seconds, we were losing 1-0. Then you are close to 1-1, and another defensive mistake and it's 2-0.
"Then you are close to 2-1 and they score, 3-0, on the counter-attack. You are close to 3-1 and they score, 4-0, on the counter-attack.
Jose Mourinho said United had to 'keep training and keep fighting.'
"You come with a strategy and you can't concede a goal in the first minute. That changed the game completely."
Mourinho said United had "wanted to create chances and score goals," adding: "In the end, I think [Thibaut] Courtois had more saves than De Gea.
"Every time they went up, they scored. But I told to the players that we should take the risk in the second half even though we know they are very good on the counter-attack."
He later told United's official website: "I have to apologise for that as the leader of this dressing room. But there is only one answer: training and keep fighting."
He said the reception he had received from Chelsea fans on his return to his old club was "fine" but added: "I was not expecting anything different. The crowd behaved in a normal way."
Sunday's defeat leaves United seventh in the Premier League, five points behind fourth-placed Chelsea and fifth-placed Tottenham Hotspur and six points adrift of Manchester City, Arsenal and Liverpool.
Mourinho said: "We play Burnley, Swansea, West Ham. We need now to win matches to close that gap."
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