PSG have included Mbappe and Cavani in their travelling party for the trip to Belgium, but Tuchel has a decision to make about how long they feature for, if at all.
Mbappe returned to action against Nice last Friday, scoring as a substitute in the 4-1 win following three weeks out with a thigh strain.
Cavani did not get off the bench in Nice, with the Uruguay star being managed more carefully given his absence of almost two months with a hip injury.
Tuchel could start both in theory, but the German insists neither can play the full 90 minutes.
"We're here with 17 players plus three goalkeepers, as usual," he told reporters on Monday. "But it's still too much for Kylian to play 90 minutes, we have to decide if he starts or ends the game.
"It's the same with Edi [Cavani]. I have a few ideas, a lot of things in my head, but I do not have the XI yet, because we still have one last training session.
"It's great that Kylian and Edi are with us. It was very long without them, but the others took the opportunity.
"Now we're stronger with them but we have to think about the best match-for-match solution. It is clear that without Ney, Edi and Kylian we lack quality and experience, but the other players proved that we were able to play at a very high level.
"But for me, we talk too much. This is the Champions League, an away game, it's the hardest in Europe."