Branislav Ivanovic admitted Chelsea's defence have lost concentration in recent matches and conceded goals too easily.
The Serbian international demanded his team must improve against Steaua Bucharest tonight in the Champions League.
Chelsea have conceded six goals in their last two matches and allowed Southampton to score after just 13 seconds at the start of the month. This has caused Ivanovic and his teammates to come under scrutiny since the start of December.
He said: "In some goals we lost concentration. We conceded a couple of goals from set-pieces as well which is not usual for us.
"We know how strong we are in that part and we just have to get back to normal."
Jose Mourinho's side have already conceded more goals in the league this season than they did in the whole of the 2004/05 championship-winning campaign. Ivanovic refused to highlight individual errors but explained they have not defended well enough.
"We change a lot of small things for every game depending on the opponent," Ivanovic added. "All these things have to work perfectly to not concede a goal.
"It's always important to not concede a goal and to keep a clean sheet. This, game by game, gives confidence for defenders and gives the stability for the team."
The Blues need a win to guarantee top spot in Group E, which would ensure they would avoid the likes of Real Madrid, PSG and Bayern Munich in the last 16.
The 2011/12 champions have had an mixed start in Europe's premier club competition this term. Victories against Schalke and Steaua Bucharest with 10 goals scored and none conceded will give Chelsea fans hope of a second success in the tournament, but Mourinho's men were defeated home and away against Basel.
Chelsea sit top of their group with nine points and a draw with the Romanian champions would be enough to top the group if Schalke draw with Basel in Gelsenkirchen.
However, if Basel claim victory in Germany, Chelsea must win at Stamford Bridge to stay top.
Mourinho's teams in the past have been based on a strong and reliable defence, and he will demand that Ivanovic and company look to recreate their early season form ahead of the Christmas period.
Write for GiveMeSport! Sign-up to the GMS Writing Academy here: http://gms.to/1a2u3KU