The Galatasaray forward, who left Stamford Bridge in June 2012,returns to London for Tuesday's second leg of their Champions League round of 16 tie.
Mourinho said: "Is he (Drogba) the same player at 36 years old as he was at 26? Nobody is, but he is certainly still one of the best strikers in the world."
Drogba said he may not celebrate if he scores against his former team-mates.
The Ivory Coast striker netted 157 goals in 342 appearances during an eight-year spell at Chelsea.
With his last kick for the club, he tucked away the decisive spot-kick in the penalty shoot-out at the end of the 2012 Champions League final against Bayern Munich.
Mourinho, who signed Drogba for Chelsea from Marseille in 2004, was given a rousing ovation when he made his return to the Blues dugout in August after a near six-year absence and hopes the Ivorian will experience similar acclaim.
He added: "He is undoubtedly one of the most important players in the history of the club. All Chelsea supporters, we agree with that."
Mourinho also believes that Drogba, who was voted by fans as Chelsea's best-ever player in the club's official magazine in 2012, will return to the club in the future.
"I think it has to happen one day," he continued. "When, I don't know. As a player, as a coach, as an ambassador, next year, in four, 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."
Chelsea will be the first English side to reach this season's Champions League quarter-finals if they win but Mourinho is taking nothing for granted against the Turkish champions, managed by former Manchester City boss Roberto Mancini.The tie is finely poised after a 1-1 first-leg draw in Istanbul.
"Everything is in the balance," Mourinho said. "I think it will be that way as we go into the end of the game.
"I don't think that one of the teams is going to make a result where people at half-time already know who is going to win."
Drogba hopes to mark his return to Chelsea with a Galatasaray victory but plans to keep his emotions in check.
He said: "With all the respect that I have for this club, for the fans and for the players, I might not celebrate, but if we win I'll be happy.
"I have nothing to prove to this club. Everything I had to prove I think I did it with Chelsea. I come here with another team.
"I just want to win the game, I just want to qualify. There's nothing personal against Chelsea, there will never be."
(BBC Sports)
