Maurizio Sarri accused his Chelsea players of over-confidence after throwing away a lead to lose 2-1 at Wolverhampton Wanderers.
Chelsea looked to be on course for victory thanks to Ruben Loftus-Cheek's early opener as the visitors dominated the first-half on Wednesday, but two goals in four second-half minutes saw Wolves secure their first win in seven games and inflict just Sarri's second defeat since taking charge in the summer.
Next up Chelsea face the daunting task of halting Manchester City at Stamford Bridge on Saturday and Sarri suggested a jolt to their confidence could be a blessing, but declared himself "worried" by the manner of their defeat at Molineux.
"After this match we are less confident but maybe it's better because sometimes my players show me that they have too much confidence," said the Italian.
"We played well for 55 minutes. After the goal for 1-1, which was an accident because we were in full control of the match, we were suddenly another team without the right distances and without our football.
"I don't know why. I am really very worried, not for the result but the fact that after the first goal we conceded we did not react very well. I am really very worried for this, not for the result."
"I don't think it depended on the changes, it depended on the mentality of the team. I saw a team without a reaction. You can lose a match but not without a reaction."
"For me, City at this moment are the best team in Europe or maybe the best in the world."