Fabregas, who previously spent eight years at Arsenal, became Chelsea's first signing of the summer when he arrived from Barcelona earlier this month.
The Spaniard fell out of favour at the Nou Camp last season but Mourinho is in no doubt of Fabregas' quality.
"He is the kind of player we need - not just for his qualities as a player but also his competitive personality," Mourinho told Chelsea's official club website.
"He is a player in the best years of his career. He knows England well, he knows the Premier League well, so he will not need time to adapt.
"He is the kind of player who will come and train for a couple of weeks and be ready to prove himself."
Fabregas is currently on World Cup duty with Spain, who are unable to make the knock-out stages of the competition after opening defeats to Holland and Chile.
Fabregas has stressed it was his decision to quit the Camp Nou for Chelsea, as president Josep Maria Bartomeu even tried to prevent him from moving.
"I asked Barcelona to find a way for me to leave the club. The president tried to stop the sale, but I already had my mind made up," he told Cadena Cope earlier this week.
"At the time, I spoke with [sporting director] Andoni Zubizarreta and the president [Josep Maria Bartomeu] and I told them I wanted to leave."