The Dutch playmaker was a key figure in an Inter Milan side which secured an historic Treble under Mourinho's tutelage in 2010.
He feels that experience could work in his favour as he now plots the continental downfall of his former boss.
Sneijder, along with ex-Chelsea forward Didier Drogba, will be able to offer Galatasaray's coaching and playing staff a greater insight into the Portuguese coach's methods - and that could significantly improve their chances of reaching the quarter-finals of Europe's premier club competition.
He said: "Mourinho knows us well and that is an advantage but the same applies for us, we know Mourinho very well, perhaps better than the Chelsea players know us.
"Chelsea is a great side, everybody has to accept that, but if we show the same spirit we did against Juventus, we could achieve another big result."
Selcuk Inan is another of the opinion that Gala can spring a surprise against Chelsea on Wednesday, with an intimidating atmosphere at the Turk Telecom Arena awaiting the visitors to Istanbul.
He said: "The way our fans support us when we're on the pitch is a driving force which pushes us to give our best. They are like the 12th man.
"Our strength is that all our players have very powerful characters.
"If we really put our heart into what we do and reflect all our quality into the game, play with our strength of character, then there isn't a single team we can't beat."