Kaka has missed the start of the Mourinho era after undergoing a meniscus operation in August, but he has now returned to action and was on the scoresheet in the 4-2 victory over Villarreal at the weekend.
Reports have suggested Kaka could be on his way back to Italy with both Inter and former club AC Milan said to have expressed an interest, but Mourinho says he will be given the chance to prove his worth at the Bernabeu.
"He will stay with Real," he told Sky Sport Italia. "It took so long to get to work with him.
"He told me, with real honesty, that he feels it is very important that he demonstrates that he is a Real player. He is now in a situation where he can do so, while last year he did not have that chance.
"Kaka is staying with us, for definite."