Mourinho was in charge of Real Madrid when they landed Varane from French side Lens in 2011, beating Sir Alex Ferguson at Manchester United to his signature.
Mourinho used Varane from the moment the Frenchman arrived in the Spanish capital and the Blues boss feels that the 20-year-old is the best young defender in world football.
"I think he is the best young central defender in the world. Already I think he's the best," he told Yahoo.
"A young player must be somebody who plays in the World Cup for the first time.
"These are players like Hazard, Neymar and Rafa Varane. Maybe I forget some others."
Some reports have claimed Mourinho could be preparing a summer bid for Varane, but Chelsea do already have St Etienne defensive starlet Kurt Zouma arriving in the summer having struck a deal for him in January.