Deschamps knows United are overwhelming favourites for their clash.
He told The Sun: "I believe we can beat them and knock them out. If I don't believe it how can I tell my players to believe they can do it?
"It's a massive ask for us because United's objective is not to beat us but to win the competition.
"United defend very well and have enormous talent but I've spotted some weaknesses at the back. I won't tell you them but we will not be content with just defending in the two matches.
So should Manchester be afraid?
Deschamps laughed: "Ha ha. It's better for us if they are not afraid. It's better they come here certain they will qualify.
"Better they come to the Velodrome knowing it will be easy and calm - but that will not be the case. If United knock out Marseille it's normal. But what will be unforgettable is if Marseille eliminate United."