The 21-year-old attacker was this week ruled out for around three weeks after suffering an ankle injury in his side's Coupe de France final victory over Saint-Etienne last Friday.
PSG's Champions League campaign will resume on August 12 when they take on the Serie A side in the quarter-finals of the competition, and Tuchel does not expect the France star to be available for selection.
"This will have an important influence for us, but I hope that the solution will be found," Tuchel told reporters ahead of Friday's Coupe de la Ligue final against Lyon. "We still hope [he can play against Atalanta], but the chances will be very, very small. Every day counts to achieve a miracle, but tomorrow we will have to adapt without Kylian."
Tuchel's team will call on Mauro Icardi to line up alongside Neymar for the matches against Lyon and Atalanta, and Tuchel is confident the Argentine forward, who has scored 20 goals in all competitions since joining from Inter initially on loan then on a permanent transfer, will continue his good form.
"He was very strong in the first friendlies, he is always very strong with Neymar, Kylian and Angel Di Maria. He likes to play with all three," Tuchel said.
"He matches the characteristics of the team. It is not necessary for him not to touch the ball too many times. He can wait. He has a lot of patience. It's up to everyone to improve, not just him. He will improve and he can play better. He works a lot defensively, he is disciplined, he is important for us.
"If we don't play enough for him, we can tell that he is not in good shape but it is not like that. He remains the same player as when he arrived. He always wants to score and it is also our goal to give him good balls to score. He remains important for us."
Nevertheless, the German manager will not be treating Friday's final as a warm-up for the European clash, as their Italian opponents have a completely different playing style.
"It is not possible to tactically prepare for Atalanta in a match against Lyon," he added. "I don't know a team that plays like Atalanta. We can just work on our state of mind and show our hunger to win. This will prepare us from this point of view if we get a good result tomorrow."