The Dutch international, who himself moved to the Italian capital from PSV Eindhoven for £15m this summer, believes that the Frenchman has transformed the team into title contenders.
The three-time winners lead Inter Milan by two points at the top of the league having won their opening five games of the season.
"Juventus remain the favourites for the Scudetto, but Roma are there too," the 23-year-old told Sky Sport Italia.
"I was so impressed by the way we celebrated the win at Sampdoria last night, all together including the kit staff. Coach Rudi Garcia came in and improved everything that wasn't working."
Roma welcome Bologna to the Stadio Olimpico on Sunday.