Cavani, who has yet to make his United debut, has not travelled with the squad to Paris. He missed Sunday's 4-1 victory Newcastle United following a period in isolation because of COVID-19 guidelines.
The 33-year-old left PSG in the summer after seven seasons where he became the club's record scorer with 200 goals.
"He wrote part of our club's history, he is definitely a great player but it's not a story about PSG versus Edi or vice versa," Tuchel told a news conference on Monday. "When he scores he has the confidence and is one of the best No. 9s in world football.
PSG, last season's beaten finalists, will be without Marco Verratti and Leandro Paredes for United's visit, the first time the two teams have met since Ole Gunnar Solskjaer guided his side to an unlikely 3-1 win in March 2019 to book a place in the Champions League quarterfinals.
Solskjaer will be able to recall Anthony Martial despite the forward's domestic suspension but Phil Jones, Sergio Romero and Marcos Rojo will all be left at home after the trio were not included in the squad submitted to UEFA.
"Our memories against Manchester, obviously we don't have very, very good memories," Tuchel added. "We had a good result at Old Trafford but in the end it wasn't a great outcome for us.
"We had a very great run last season for the Champions League and now it's a challenge not to think too much about last season and two seasons.
"We have to overcome these difficulties, show what we are able to do now and start this new Champions League with zero points."