After drawing 0-0 with Manchester City in Sunday's Premier League match, Ruben Amorim's men will travel to the Groupama Stadium for Thursday's first-leg game.
Despite the barrage of criticism following his occasional blunders, the Cameroon international remains confident heading into the European tie.
"We all know the importance of this game, we will go there with the winning mentality," Onana told reporters.
"We have to go there and show who we are. If we are focused, stay compact and together and follow the gameplan we will be winning the game. Of course it’s not going to be easy but I think we are way better than them."
Manchester United have won only three of their last 10 matches in all competitions and currently sit 13th in the Premier League.