PSG spent a world-record fee of €222 million on Brazil star Neymar and then brought in Mbappe - the star of Monaco's Ligue 1 title triumph last term - on a season-long loan, with an expensive permanent transfer to follow next year.
But former PSG coach Ancelotti, who returns to Parc des Princes in the Champions League on Wednesday, believes the French club need to give the new signings time to settle, despite their quality.
"With Neymar and Mbappe, Paris are looking for an identity," he told Le Figaro. "At Bayern, it's clear. For years, the line has been drawn, the identity is clear, which is not yet the case at PSG.
"But Paris are already at the level of the best. When you buy players at this price and this level, you have to have time and they have to enter the mindset of the team. That is their challenge."
Ancelotti is excited to return to PSG, who he believes have developed into a "big club" since his 2013 departure.
"[The feeling is one of] excitement - I will find a Paris Saint-Germain which has changed a lot since I left in 2013," he said. "The club has improved and has more experience after a lot of investment. Now it's a big club.
"I am happy to see the Parc des Princes, the supporters and players like Thiago Silva, [Adrien] Rabiot, [Thiago] Motta, [Marco] Verratti, [Javier] Pastore.
"We had a good relationship, they missed me. I also keep in touch with the president [Nasser Al-Khelaifi]."
He added: "I have good memories of my time there. I helped the club to change mentality and PSG managed to grow a little more afterwards.
"Even if we had problems following my departure for Real Madrid, when I think of Paris, I am happy."