Mbappe had been expected to miss the quarter-final tie against Atalanta but Tuchel has suggested that he could feature in Lisbon as a substitute.
Speaking in his pre-match press conference, the German coach said: "If he has a good training session today and nothing extraordinary happens, he will be with the team tomorrow.
"We are very happy that he is with the group and that we can have the possibility of finishing the match with him."
The 46-year-old manager had previously said that it would require a "miracle" for Mbappe to be involved in the quarter-final after suffering an ankle injury in the French Cup final last month - although he is confident that Neymar should be able to carry PSG's attack if Mbappe is unable to play.
Tuchel said of the Brazilian striker: "I have the impression that there is always a lot of pressure on him, but he likes it.
"We are very happy that Kylian is also with the group and that we can have the possibility to finish the match with him, because the two like to play together. Neymar is decisive on the big matches and I am convinced that he can be the key man for us. He has the necessary quality for this kind of situation."
Tuchel has been left worried by the fitness of midfielder Marco Verratti, who didn't travel to Portugal as he is suffering with a calf injury.
He said: "There isn't much time, but we will try everything."