Mourinho and his side will come up against former Chelsea striker Drogba at Stamford Bridge with the last 16 clash evenly poised after a 1-1 first leg draw in Turkey.
The Ivorian is Chelsea's fourth highest scorer in the club's history and he is expected to receive an excellent reception in west London.
"Undoubtedly (Didier Drogba) is one of the most important players in the history of this club," he said.
"That is not a doubt. We all, as Chelsea supporters, agree with that. We don't say the most important one because that is not fair.
"Is he the same level as when he was 26? Nobody is. But he is one of the best strikers in the world, that's for sure."
Galatasaray came from behind to salvage a draw in the first leg, but will need to raise their game if they are to beat last year's UEFA Europa League winners.
"If you look at the (first leg) one team was better in the first half, another was better in the second half," he added.
"I believe the result will go until the end. I don't believe one of the teams is going to make a result where everyone knows at half-time who is going to win.
"For me (Sneijder) is one of the two or three best number tens in the world. It is as simple as that."
"The game will be tough in general but we're playing at home, we'll be on the front foot and hopefully it goes well," Cahill said.
"I admire Drogba as a guy, he's a fantastic players and playing against him is going to be tough. He has a lot of quality."
