The Netherlands midfielder won the prestigious trophy under Mourinho in his time at Inter Milan, and he has repeatedly admitted that he still has a great relationship with the Real Madrid boss, describing him as a "second father" at the start of this week.
But Sneijder, who also spent two years at Estadio Santiago Bernabeu, insists he will be doing his old boss no favors when the sides clash Wednesday.
"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 kickoff tomorrow night," Sneijder 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 Sneijder 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," Sneijder said.