Mourinho will be reunited with his former player when Galatasaray visit Stamford Bridge in the second leg of their Champions League Round of 16 tie on Tuesday.
The Ivory Coast international spent eight seasons with Chelsea and his send-off in the summer of 2012 was the winning penalty in the Champions League final win over Bayern Munich.
"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," Mourinho told Sky Sports News. "But we think it is not the right moment to speak about him.
"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.
"Undoubtedly he is one of the most important players in the history of this club - that is not a doubt. I think all Chelsea supporters we agree with that.
"We don't say the most important one because that is not fair for other people that were in the same level and in the same generation, but for sure we all agree one of the most important players in the history of our club.
"Is he the same player at 36 that he was at 26? I think nobody is, but he is one of the best strikers in the world that's for sure."
The match also sees Mourinho's predecessor as Inter Milan boss return to English football.
The former Manchester City manager Roberto Mancini has spoken of his willingness to go to dinner with Mourinho if his Gala side win Tuesday's match.
However, when asked whether he would be happy to have a post-match meal with the Italian, Mourinho said: "No, because I have no interest. After the match I don't do things because I win or because I lose.
"After matches what I have in my mind to do, I do. It doesn't matter if I win or if I lose.
"And in my plans after the game, I don't have a meal with somebody that has the same job as I have and this is the only thing we have in common. The fact we both are football managers. Nothing else."