The Brazil international was continually linked with a return to AC Milan throughout the summer, but the move did not materialise because of financial reasons.
"Neither Kaka nor anyone else is a dead weight," Mourinho stated at a press conference.
"Against Granada we went in another direction, [Karim] Benzema, [Jose] Callejon, and [Luka] Modric played and we also wanted to give some rest to those who played against Barca.
"The three who were left out were the first choices, but there will be a time when Kaka will be an option once more."
The 30-year-old midfielder has yet to feature in an official game for Madrid this term.