Maurizio Sarri accused his Chelsea players of over-confidence after throwing away a lead to lose 2-1 at Wolverhampton Wanderers on Wednesday.
Chelsea looked to be on course for victory thanks to Ruben Loftus-Cheek's early opener as the visitors dominated the first-half, 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."
Chelsea missed several chances to extend their lead and were denied a possible penalty early in the second half when Alvaro Morata tangled with Wolves defender Willy Boly.
Yet Sarri refused to discuss the penalty appeal and insisted his decision to make five changes also wasn't the cause of his side's defeat.
"In the first 10 minutes of the second-half we could have scored four or five goals. We had four goal opportunities but I don't want to discuss those or the referee," he added.
"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."
Chelsea are now 10 points off City at the top of the table, but of more immediate concern are now only in the top four above Arsenal thanks to their superior goal difference.
"Manchester City are in another category, said Sarri. "We have to play and fight for the top four. It is a very difficult result for the top four.
"For me, City at this moment are the best team in Europe or maybe the best in the world."
Chelsea took an 18th-minute lead when Loftus-Cheek cut in from the left and hit a curling shot that struck the head of the unlucky Conor Coady to wrong-foot goalkeeper Rui Patricio and creep inside the near post.
However, on his first Premier League start, 18-year-old Morgan Gibbs-White lead the fightback as he fed Raul Jimenez, who drilled a low shot under goalkeeper Kepa Arrizabalaga.
Moments later Molineux erupted when Matt Doherty crossed and Diogo Jota side-footed home for the winner.
"He is one of the best generations of English players for a long time," claimed Wolves boss Nuno Espirito Santo of his midfield starlet.
"He is a reality of the fantastic job that national teams are doing, so well done to them."