The Brazilian midfielder signed for Los Blancos from AC Milan in 2009 and has made 105 appearances, scoring 26 goals.
"At Real Madrid we never saw the real Kaka," Mourinho told Rai. "He must be happy and Milan could be a good solution, but he is a Real Madrid player."
Ahead of his 50thbirthday, the former Chelsea and Porto coach also talked about his future in coaching and a possible return to an old club.
"Now I am 50, I could still coach for at least more 20 years and many things can happen. Maybe [I could] come back to Inter, why not?"
Mourinho also spoke about the Champions League campaign and focused on Juventus as a particular threat, although his team is set to face Premier League leader Manchester United in the last 16.
"Juventus are a top team," he said. "It's easy for them to say that they are not the team to beat in the Champions League, [and although] a lot of people say that the favorite teams are Real Madrid, Man United and Barcelona, I say that Borussia Dortmund, Bayern and Juventus can win the Champions League."
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