The 36-year-old, who will come to the end of his Galatasaray contract in the summer, ended an eight-year spell at Chelsea by scoring the winning penalty in the Champions League final in 2012.
This evening, the Ivory Coast international will return to Stamford Bridge for the second leg of the clubs' last-16 tie in the Champions League.
"He is a Galatasaray player, we know he is a free agent and we know he finishes his contract at the end of the season, but we think it is not the right moment to speak about him," Mourinho told Sky Sports News.
"I think it has to happen one day. When, I don't know - as a player, as a coach, as an ambassador - next year, in four or five years, in 10 years, I don't know.
"But when a person represents so much to a club and when a club represents so much to a person, and that's the case, I think he has to be back one day."
The tie currently stands at 1-1 following the first leg in Turkey.