Mbappe was at Monaco at the same time as Ranieri during the 2013-14 season and last summer completed a blockbuster loan move from the Stade Louis II to the capital that will be made permanent for an initial fee of €145 million in the summer.
The 19-year-old has continued his stellar progress at PSG, where he has thrived beside Neymar, and Ranieri believes he is capable of becoming the best in the world.
"He’s fantastic – if he continues to mature, he can will the Ballon d’Or," the Italian said. "At the same age, the only equivalent in France is the great Thierry Henry."
Ranieri did confess, however, that he was unable to really appreciate the talent of a player who was only a boy when they crossed paths in the principality.
"I had seen Anthony Martial play for Lyon’s reserves, but Mbappe was too young at the time," he admitted.
Ranieri’s Nantes clash with PSG on Sunday in a Ligue 1 encounter, having lost 4-1 to the runaway league leaders earlier this season.