The midfielder's performances have been rewarded with a recall to the Brazil squad.
Mourinho said: "I've known the real Kaka. He was a fantastic player when I was in Italy.
"Real Madrid have never had that version of him and he's finally found physical balance above all. He enjoys playing football. He is strong and is good at changing the pace to the game. It's only natural for him to return to the Brazilian national team and I'm happy for him because he's earned it through hard work. Football should rejoice at Kaka's return."