Sami Hyypia's side host PSG in the first leg of their clash on Tuesday as they aim to progress to the quarter-finals for the first time since reaching the final in 2002.
Leverkusen have enjoyed an impressive season and sit second in the Bundesliga, but head into the visit of the defending Ligue 1 champions on a run of five defeats in their last seven matches in all competitions.
And sporting director Voller accepts that PSG, who sit five points clear at the top of the French top flight, will be expected to reach the next round.
"PSG are obviously the favourites," Voller told L'Equipe. "We will play a game against a team that is one of the five best in Europe. Bayer has nothing to lose."
Leverkusen are 16 points behind runaway Bundesliga leaders Bayern Munich after 21 games, and were eliminated in the DFB-Pokal quarter-finals by second-tier outfit Kaiserslautern last week.
Yet Voller is adamant that the club's hierarchy are pleased with how the campaign has progressed so far.
"We're pretty happy with what we are doing since the beginning of the season," the former Germany international added.
"We are second in the championship behind Bayern Munich, which is the best team in the world, and ahead of Borussia Dortmund."
