Messi, 34, has had just two training sessions since joining PSG as a free agent on Wednesday after Barcelona were unable to complete his new contract. He won 10 LaLiga titles and four Champions League trophies during a 17-year long stint at Barcelona.
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The Argentina captain will be presented along with the other summer signings to the fans at the Parc des Princes on Saturday ahead of the encounter but will not play.
Asked when Messi, who last played in a 1-0 victory over Brazil in the Copa America final on July 10, will make his debut, Pochettino said on Friday: "We have to take it one step at a time. The priority is that he feels well. A month ago, he played the Copa America final and he has to make his debut in the best conditions. But I've seen a very matured Messi, the player we all know. What I see is a Messi that is happy and with incredible energy."
Messi admitted he did not know when his first game with PSG will be.
"We are not a team yet," he said. "We have a team with players that shine on their own and the tremendous challenge we face is to make with those names a good team. I face this challenge with a lot of enthusiasm and energy. We want to have a team not only for social media but one that covers the day-to-day demands.
"As a coach you always want to have that problem of creating a team with [surplus of] top players and take decisions, that is where we are here for."
Pochettino, meanwhile, has brushed aside rumours of Mbappe's possible departure.
Mbappe has not signed a new deal with PSG, with his current contract ending next summer, and Messi's arrival has increased speculation that the Frenchman could leave.
"Kylian is our player and will continue to be," Pochettino said.
PSG confirmed on Friday that veteran defender Sergio Ramos will be out of action for four weeks.
The former Real Madrid captain, who joined PSG as a free agent this summer, sustained a calf injury in preseason training and will be unavailable until after the international break in September.
PSG surrendered the French league title to Lille last season and were eliminated in the Champions League semifinals by Manchester City despite boasting the likes of Neymar, Mbappe and Angel Di Maria in their ranks.
Adding Messi to the fold will have raised expectations among fans and the board at the Qatari-owned club, whose management have invested heavily in a bid to see the club win their maiden title in Europe's premier club competition.