Mourinho said: "He's just started training a couple of days ago - with the group and then on his own.
"He's not ready to play. He dominates in the air and has the physique to adapt to the English way of playing. Maybe one of the reasons we lost the championship last season was because we did not have a lot of defenders.
"We lost Robert Huth and William Gallas in pre-season and John Terry was out for three months. Khalid Boulahrouz and Ricardo Carvalho were also out injured and we did not really have any solutions.
"I was scared whenever any of my central defenders went into a challenge and it was not a good situation for us just to have Carvalho and Terry.
"Now there is no reason to be scared like we were last season because we have Terry, Carvalho Tal Ben Haim and Alex."
The Chelsea boss also believes competition for places among his defensive quartet will be something the Brazilian will relish.
He added: "He plays in the most famous national team in the history of football and if he can cope with the challenge of that, he can cope with it at Chelsea.
"It is competitive in the Brazil squad and in Chelsea it is the same."