Big things were expected of the Brazilian forward when he made the switch from AC Milan in 2009, but injuries and loss of form have left Kaka a peripheral player in the Madrid squad.
He was linked with a return to Milan over the summer, a move which failed to materialise, while the arrival of Luka Modric from Tottenham has further reduced Kaka's chances of winning a place in the side.
Yet Mourinho insists the 30-year-old still has a part to play for La Liga's champions and should not give up hope.
"What has happened to Kaka has happened before to many great players," the Portuguese told AS.
"For one reason or another they have failed to continue their career at a new club when they could do fantastic things before moving.
"Players are not robots. Kaka came here with fitness problems which troubled him throughout his first season.
"In his second year he had surgery and needed a few months off. When he returned he did so bit by bit.
"Last season there were times when he was very, very good. He made some important contributions, for example when (Angel) Di Maria was injured.
"I said before the transfer window closed that Kaka was not a problem for Mourinho. He knows because I told him things are not easy in his situation, and he has accepted the challenge.
"He is training well. When we consider how we should use him we will try and use him as best we can.
"It will not be easy but I'm sure Kaka will get his chance. Tiredness, dismissals... he will get a chance.
"And I am convinced that when the time comes, Kaka will respond and help Madrid."