Mourinho's men finished third in the league last season, four points adrift of champions Manchester City.
The Londoners 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 the signing of former Arsenal captain Fabregas is the biggest for Mourinho, who believes the midfielder is just what Chelsea were missing last season.
"He can play as a number eight or number 10, plus the fact he has experience in the Premier League," Mourinho said.
"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."
