Mourinho said that Chelsea have bought a player in his prime and he should be able to pick up where he left off when he departed England and Arsenal in 2011.
"He is the kind of player we need," Mourinho told ITV Sport. "Not just for his qualities as a player but also his competitive personality.
"He is a player in the best years of his career - 26, 27, 28. He knows England well, he knows the Premier League well, so he will not need time to adapt.
"I think he is the kind of player who will come and train for a couple of weeks and be ready to prove himself."
Fabregas played 212 league appearances for Arsenal between 2003 and 2011.