The Netherlands star insists his side will be gunning for victory at Santiago Bernabeu in the quarter-final first leg on Wednesday
The Netherlands midfielder won the trophy under Mourinho in his time at Inter, and has repeatedly admitted that he still has a great relationship with the 50-year-old, describing him as a "second father" at the start of this week.
But Sneijder, who also spent two years at Santiago Bernabeu, insists he will be doing his old boss no favours when the sides clash at Santiago Bernabeu.
"It will be a very good game. It is very exciting for me to be back here. This is the highest level and we cannot wait for the kick-off tomorrow night," he told reporters.
"I am happy now and am confident. The only thing I can say about Mourinho is that it was fantastic playing under him but tomorrow we are enemies."
The 28-year-old also admitted that the Turkish champions will be pushing for victory, refuting suggestions they will seek to defend prior to the return leg in Istanbul.
"It is very important to score an away goal but it is also important not to lose. We are not here to defend, we are here to play our game. We are very confident. We are here for victory," he went on.
"It is no surprise that we are in the quarter-finals. I am very happy with Galatasaray. I felt very welcome from the moment I arrived. It is an amazing club."