While serving a nine-month ban for his clash with a referee in Ligue 1, Fonseca is free to take the bench in Europe.
Ahead of their Europa League quarterfinal first-leg clash tonight in Lyon, the Portuguese said: "We know it will be an important match for Manchester United, but it will also be a very important match for us.
"Perhaps even more decisive for them, because it's their only chance to secure a place in Europe. For our part, we prepared for this match as if we were going to face the best Manchester United possible."
He then added: "I understand the players' ambition, and I'm very happy about it, but as a coach, I have to maintain a certain balance. We want to win, obviously, but we also know that we're going to face one of the best teams in the world. This balance is essential right now.
"We'll have opportunities, but we'll have to stay clear-headed against a very high-level opponent. We have a lot of faith in our team, in our quality. We're going to play with the ambition to win. I sense a real motivation in the squad. We'll be playing at home, with the support of our fans. We want to give everything to win, and I feel that the team is ready."