The Ligue 1 outfit go into the game trailing 2-0 after last week's meeting between the two sides at Stade Velodrome, but the France international is refusing to throw in the towel just yet.
"It is not an ideal situation," Mandanda told reporters during a press conference in Munich on Monday. "But it's a football game.
"One must believe because there have been miracles in football [before]. We believe in our chances.
"I hope that tomorrow [Tuesday], we give 100 per cent and do not have any regrets [afterwards]. Then we will see what happens."
Meanwhile, Marseille coach Didier Deschamps acknowledges that Bayern have one foot in the semi-finals, but he is confident that his players will rise to the occasion.
"We will have to mobilise all our forces," he declared. "And it is the game itself which will mobilise the team.
"It is a historic opportunity for us. I did not have to find many words; everybody knows what is at stake.
"Bayern are close to the semis, but we have to take the field believing in ourselves. We need to have a strong mentality.
"Our quality is still high. We must enjoy it, even if, after the first leg, the challenge is not at all easy.
"We must go to the very maximum and play in a focused, intelligent way from the very first moment to the last."
Marseille's meeting with Bayern in Munich gets underway at 20.45CET on Tuesday.
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