Villas-Boas: Chelsea will put things right
The Londoners have lost three of their last four Premier League matches including two in a row at home against Arsenal and Liverpool.
"I think we are playing quite well overall but not getting the results and I need to analyse the details of what is going wrong at the back," the under-fire Villas-Boas told reporters.
"We have a good squad, we are still in four competitions and we will put it right."
Chelsea are top of Group E with eight points from four games, two ahead of Leverkusen. Valencia, who take on Belgians Racing Genk on Wednesday, are third on five points.
Villas-Boas said he needed to sort out their defensive vulnerability after getting beaten 5-3 by Arsenal three-and-a-half-weeks ago and going down 2-1 to Liverpool on Sunday.
However, he added that the controversy surrounding skipper John Terry, facing police and FA investigations over alleged racist remarks made to Anton Ferdinand of Queens Park Rangers last month, was not an issue.
"I don't think its affecting what he does on the pitch," said the Portuguese.
Chelsea have not won an away match in the group stage of the Champions League since a 2-0 victory at Spartak Moscow 13 months ago and were held 1-1 at Valencia and Genk this season.
Defeat in Leverkusen, coupled with a win for Valencia over Genk, would make the London club's final match at home to the Spanish team on December 6 a tense affair.
"Chelsea are under pressure," said Leverkusen captain Ballack of his former club.
"This is a crucial game. We have won our home games against Genk and Valencia but we will need to be in top shape," added Ballack who scored in a 2-0 Bundesliga victory at Kaiserslautern on Friday.
Bayer Leverkusen: 23-Bernd Leno; 5-Manuel Friedrich, 21-Omer Toprak, 24-Michal Kadlec, 27-Gonzalo Castro; 6-Simon Rolfes, 8-Lars Bender, 13-Michael Ballack, 18-Sidney Sam, 9-Andre Schurrle; 11-Stefan Kiessling
Chelsea: 1-Petr Cech; 17-Jose Bosingwa, 2-Branislav Ivanovic, 26-John Terry, 3-Ashley Cole; 7-Ramires, 16-Raul Meireles, 8-Frank Lampard; 10-Juan Mata, 9-Fernando Torres, 23-Daniel Sturridge