Ex-Arsenal captain Fabregas has hit the ground running at Stamford Bridge since crossing from Barcelona for a reported fee of £30 million in June.
The Spain midfielder has provided four assists in Chelsea's first three Premier League games, helping them to the top of the embryonic table.
Mourinho says Fabregas' form has come as no shock, pointing to the player's experience and happiness at Chelsea.
"He doesn't need to adapt," Mourinho said.
"This is a league that he knows, that he had played in for years (with Arsenal). He was moulded there.
"When he came to Chelsea he was very happy to come back to this league.
"When a boy is happy, he doesn't suffer any kind of pressure, he knows that his coach trusts him, knows that his mates admire him, feels important – [his form is] not a surprise to me."
