Chelsea are in France for the second-leg of their Champions League round 16 tie defending a 2-0 lead.
David told L'Equipe: "The task will be very difficult. Especially because Renato (Sanches) is injured. It impacts us because he is a very important player.
"But, in football, anything can happen, it can change in a quarter of an hour. We will never give up.
"Personally, that nobody believes it, it doesn't really affect me. The most important thing is that I believe in it. And that all my teammates too. Because it is up to us to do this job."
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