Jose Mourinho took responsibility for Chelsea's complacency but also pointed the finger at the officials after a 1-1 draw at Champions League minnows Maribor.
The Blues remained unbeaten this season, but could have increased their advantage at the top of Group G to five points with two games to play had they won.
Eden Hazard's late penalty was saved, meaning Mourinho's men must now win in Gelsenkirchen later this month against a Schalke side managed by Roberto Di Matteo - the man who led Chelsea to the 2012 European Cup - to ensure progress to the last 16 with a game to spare.
Chelsea had beaten Maribor 6-0 two weeks ago and came to life only after Agim Ibraimi's goal, Nemanja Matic's close-range strike salvaging a draw.
Maribor 1 Chelsea 1 - player ratings
Maribor 1 Chelsea 1 - player ratings
1/11 Petr Cech
Busier night than he would have anticipated – did a solid job and could do nothing about Ibraimi’s stunning strike. 6
2/11 Branislav Ivanovic
Not as dominant as usual – switched into an unfamiliar position on the left during the second half. 5
3/11 Kurt Zouma
Made a crucial clearance off the line early on but Chelsea’s defence looked far more rugged than usual without Gary Cahill. 6
4/11 John Terry
Struggled as the Chelsea back-four was left exposed on a number of occasions but made a significant contribution at the other end as headed Fabregas’s corner into the path of Matic. 6
5/11 Filipe Luis
Unconvincing display from one of last season’s Champions League finalists – will lose his starting place to Cesar Azpilicueta when Chelsea face Liverpool at Anfield on Saturday. 5
6/11 Nemanja Matic
Although he scored, Matic failed to provide Chelsea with the sort of security Mourinho’s men tend to enjoy in the centre of the field. 6
7/11 Cesc Fabregas
Continues to orchestrate the Blues band of attackers – opened up the Maribor defence on numerous occasions. 7
8/11 Andre Schurrle
Relieved to see an offside flag as he spurned a sitter from point-blank range and then withdrawn at half time. 5
9/11 Eden Hazard
Caused the Slovenians all sorts of problems, as you’d expect him to – but missed a penalty and a very easy chance to give Chelsea victory late on – he’ll be disappointed. 6
10/11 Didier Drogba
Typically robust performance from the Chelsea legend but he lacked finesse on this occasion. 5
11/11 From the bench: Oscar
Showed some nice moments after his half-time introduction, a delightful one-two with Hazard that led to a great opportunity for the Belgian stood out. 7
"Many times surprises happen when people are not fully focused, when they believe the game can't be a difficult one," Mourinho said.
"I was conscious of it, but I was not successful in passing over that message.
"If I cannot convince the players that the game is difficult and you have to play from minute one, obviously it's my responsibility.
"Only when Maribor scored did the players realise the game was in danger. After that, a fantastic reaction. We deserved to win, clearly."
Mourinho felt Diego Costa, brought on at half-time, had a strike unfairly ruled out for offside.
"We scored two goals, two very good goals," he added.
"The second goal, the referee (Italian Daniele Orsato) decided to disallow but it's a clear goal.
"Normally we should win because we scored two goals. But we have to accept it.
"We didn't reach the number of goals that we should have in the second half, but the team played well. And we scored a great second goal.
"And the referee's team did not have a good night."
The Slovenians were unrecognisable from the side beaten at Stamford Bridge, but also indebted to Jasmin Handanovic for a couple of outstanding saves.
It is clear Chelsea have work to do ahead of Saturday's Premier League trip to Liverpool, but Mourinho was happy with the reaction to conceding.
He said: "After that I thought we played magnificently: creative, fast, good dynamic, lots of chances to score. By that, we deserved to win."
Mourinho had introduced striker Costa and Oscar at half-time, hauling off Willian and Andre Schurrle.
The Blues boss was critical of his players after the Capital One Cup win at Shrewsbury and the Premier League defeat of QPR, but he held his counsel after the latest insipid display.
"I tried to improve my team," he said.
"When you try to improve your team, sometimes it's for tactical reasons. Other times it's for individual performances.
"I think I was successful with the changes I did. In the second half the team were much better than in the first half."
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