The 21-year-old joined the current Ligue 1 champions in 2012 from Pescara and has emerged as one of Italy's brightest prospects.
"I think Verratti is a very strong player, even if he is still very young," Vieri told Bein Sport.
"He's good in the defensive phase too, he has good reflexes and some very good technique.
"He has a great future ahead of him, in my opinion, and he was a great signing for Paris Saint-Germain."
Verratti has won four caps for the Italian national team.