Chelsea boss Andre Villas-Boas says he's wary of the quality of their Champions League group rivals.
Chelsea should top their group on paper.
But he said: "It's an extremely difficult group in my opinion.
"We all saw what Valencia did last season and they are very, very regularly in the Champions League.
"The Belgian champions (Genk) have lost a couple of talents that they had with (Thibaut) Courtois, who joined us and went on loan to Atletico (Madrid), but have some good talent available."
He added of Tuesday's opponents, who reached the Champions League final in 2002 and have former Chelsea star Michael Ballack back in their ranks: "Leverkusen have this strong emotional impact of being present in the Champions League final and always wanting to perform well.
"They lost (Arturo) Vidal, of course, to Juventus but have again good quality players."
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