PSG's Uruguayan striker Cavani has been sidelined after sustaining a hip muscle injury in Saturday's 1-0 Ligue 1 victory over Girondins de Bordeaux and PSG are already without Neymar, who was ruled out with a foot injury.
The absence of the world's most expensive player, Neymar, will put the spotlight on the world's most expensive teenager, Kylian Mbappe.
While Tuchel said that it is unfair to expect Mbappe to make up for the injured players.
"It’s not really his job to replace Cavani and Neymar, and he can’t do all three things at the same time," said Tuchel.
"I want us to play with courage, play as a team, together, tight knit, fight, ready to suffer. And I think I don’t really want to talk about the result but we want to score perhaps more than one (goal)."
PSG's task against confident Man Utd is made no easier by injuries to key players, with forwards Edinson Cavani and Neymar sidelined as well as right-back Thomas Meunier.