Chelsea captain Cesar Azpilicueta apologised to the club's fans after Manchester City's 6-0 win at the Etihad Stadium on Sunday.
City's demolition of the Blues was their heaviest defeat in the Premier League, making it four away losses in a row in all competitions for Maurizio Sarri's side.
Sergio Aguero hit a hat-trick, Raheem Sterling scored twice and Ilkay Gundogan was also on target as City ran riot to reclaim top spot from Liverpool.
Chelsea slipped to sixth place on goal difference and are a point behind Manchester United in fourth as the race for Champions League qualification continues.
The Blues have now lost three straight Premier League away games by an aggregate 12-0 scoreline and defender Azpilicueta acknowledged they were far from good enough.
"It is one of the worst nights in my career. It is difficult to explain what happened," he said to Sky Sports.
"The first five or six minutes went well but that is not enough. It is very frustrating and disappointing. We are conceding a lot of goals and we cannot accept that.
"When you lose 6-0 we have to accept we didn't play well and made errors. We knew it would be tough.
"When we won at Stamford Bridge [a 2-0 victory in December] it was because we played well and were together. Today was not the case.
"All I can do is apologise to the fans because it is not acceptable."
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