Chelsea defender Gary Cahill has warned his team-mates they must cut out their habit of making careless mistakes if the Blues are to make a serious bid to win the Champions League.
Jose Mourinho's side face Steaua Bucharest in their final Group E fixture at Stamford Bridge on Wednesday still smarting from a surprise 3-2 defeat at Stoke at the weekend.
Chelsea have already qualified for the last 16 of the Champions League, but a victory against Romanian minnows Steaua is essential to ensure Chelsea finish top of group, and in theory secure an easier draw in the knockout stages.
To do that they will need to be far more steady at the back than in recent weeks.
The loss at Stoke not only damaged Chelsea's chances of winning the Premier League, but also exposed a worryingly soft defence that has now conceded six goals in just two matches.
In Mourinho's previous spell in charge, the Blues boss prided himself on sending out teams with supremely-disciplined defences that were rarely breached and hardly ever due to sloppy play.
But the current defensive crop have been far less solid of late and England centre-back Cahill believes that is down to individual errors.
"Losing games is a concern. We've kept a lot of clean sheets this season, but in the last two games we've let too many goals in. That's unlike us and it's down to individual mistakes," Cahill said.
"We talked about it after Sunderland and there were unfortunate incidents at the set-pieces.
"We've had a look at it and they were not major mistakes. Maybe it's just a bit of bad luck but obviously, away from home, we've got to keep it tight.
"I feel we've done that most of the season, but this result was poor for us."
Chelsea have now conceded more goals in the league than in the whole of Mourinho's first season in charge in 2004-05 when they won the Premier League.
Set-pieces have been a particular concern, and goalkeeper Petr Cech was culpable for Peter Crouch's goal at Stoke.
Mourinho knows Chelsea must become tougher at the back to thrive in the latter stages of Europe's elite club competition, yet he needs to strike a balance between sending out defensive-minded teams and ensuring he doesn't dilute the talents of playmakers like Eden Hazard.
The Belgian has been in fine form and underlined his class with two goals and an assist in the 4-3 win at Sunderland last week.
Mourinho is still not completely satisfied with Hazard however and he said: "Eden has to try to improve the way he starts every game - the focus, the ambition, the mentality. If he does that, the potential is very good," Mourinho said.
"He needs to become comfortable with big responsibilities, this is a quality of the big players.
"They know they are better than the others, so they accept the responsibility of being better than the others.
"When the team is not playing well enough to win the match, the big players know the team is waiting for them to resolve the problems.
"The football world is full of teams where an individual can make the difference when collectively you cannot do it.
"He's a fantastic player who can only improve, but if he can feel comfortable with his responsibility to be a special player that can make a difference for us and for him."