Jose Mourinho accused his Chelsea players of complacency after they were held to a 1-1 draw by Slovenian minnows Maribor in Champions League Group G.
The Blues went behind to Agim Ibraimi's goal but Nemanja Matic equalised before Eden Hazard missed a penalty.
"Many times surprises happen when people are not fully focused, when they believe the game can't be a difficult one," said Mourinho.
"Only when Maribor scored did the players realise the danger."
|Chelsea have not lost a competitive match since the semi-final of last season's Champions League in April|
|The Blues have won just one of their last six away games in the Champions League|
|Chelsea conceded for the first time in 230 minutes away from home in the competition|
The result leaves Chelsea three points clear in Group G, needing to win one of their remaining two matches to guarantee qualification to the knockout phase.
Chelsea had thrashed Slovenian champions Maribor 6-0 at Stamford Bridge in their previous Champions League game.
And despite having talked up his opponents in the pre-match news conference, Mourinho admitted he must take some of the blame for his players' attitude in a lacklustre first half.
"If I cannot convince the players that the game is difficult and you have to play from minute one, obviously it's my responsibility," he said.
Mourinho also expressed his frustration with Italian referee Daniele Orsato and his fellow officials.
The Blues were denied a penalty early in the second half when Oscar appeared to be tripped and his fellow substitute Diego Costa had a goal wrongly ruled out for offside.
Mourinho hinted that he had history with Orsato from his time at Inter Milan, adding: "We scored two goals, two very good goals. The second goal, the referee decided to disallow but it's a clear goal.
"Normally we should win because we scored two goals. But we have to accept it.
"The referee's team did not have a good night."