Mourinho, who re-joined Chelsea from Real in the close-season, installed Varane as a regular in the starting XI at the Bernabeu last term, with the France centre-back earning praise for his composed defensive performances.
The 20-year-old has been troubled by persistent knee problems in recent times but played 90 minutes for his country in a 2-0 win over the Netherlands on Wednesday.
Reports have suggested that Chelsea could move to reunite Varane with Mourinho, who was glowing in his praise of the former Lens man.
Varane is likely to be selected for France's squad for the FIFA World Cup in June, and Mourinho believes he will go to Brazil as the best player at his position.
"I would say (Eden) Hazard for sure, Neymar, Rafa Varane," Mourinho said when asked who were the young players to watch at the World Cup in an interview after being unveiled as Yahoo's global ambassador for football 2014.
"I think he (Varane) is the best central defender in the world, a young one I think he's the best. Already, yes. I think he's the best."
One young French defender already likely to play for Chelsea next term is Kurt Zouma, who signed a £12million deal in January, but was loaned back to Saint-Etienne for the remainder of the campaign.
