The 2007 World Player of the Year joined Real from AC Milan for a fee of around 65 million euros three years ago but has mainly played second fiddle to German international Mesut Ozil this term.
The 30-year-old has shown flashes of brilliance but had a miserable end to his Champions League campaign last month when he had a penalty saved in the semi-final shootout against Bayern Munich as Real crashed out to the Bundesliga side.
"I cannot even guarantee that I will be here," Mourinho said when asked about Kaka, who has been linked in the media with a move to Paris St Germain, the Ligue 1 club coached by his former boss at Milan Carlo Ancelotti.
"I am grateful to all the players for their contribution," Mourinho added.
"He (Kaka) has played very good matches, others less good and others in which neither he nor I have come away satisfied.
"They have all participated in the league title and my way of thanking them is that all are welcome next season.
"But if one thinks that it is better not to stay and he is more interested in another project we will look at it.
"They also make their decisions and we will listen to those that have something else in their heads."