The Blues have added Fabregas, Diego Costa, Didier Drogba and Filipe Luis to their squad in the close-season after losing the likes of David Luiz, Ashley Cole, Frank Lampard and Romelu Lukaku.
But Mourinho believes the signing of the former Arsenal captain Fabregas is just what Chelsea were missing last season after finishing four points adrift of champions Manchester City in third place.
"He can play as a No.8 or No.10, plus the fact he has experience in the Premier League," Mourinho told reporters.
"He is not on the foreign list, as he counts as a homegrown player, which was important and he is not on the foreign list for the Champions League either. They were important factors.
"I see him everywhere because he can play everywhere. His best quality is the speed where he analyses the game and executes. He is difficult to shake off the ball or make the wrong decision.
"He can accelerate the intensity of the game or he can reduce it. He's very intelligent. And he has goals. He is the missing piece."