The Spanish giants take a three-goal lead to Istanbul, but the former Chelsea boss has warned his players that he has seen similar deficits overturned in the past.
"If we score a goal in Istanbul then they have got to score five, so the result tonight is very good," BBC Sport quotes him as saying.
"But I've seen a lot of things in football so nothing can surprise me. Istanbul is a very difficult place to go and they will fight hard."
Mourinho went on to say that his side were respectful of Galatasaray during the 3-0 victory in the first leg and described them as a "fantastic" team.
"We played seriously. The Spanish press were not quite so respectful towards Galatasaray but we were very respectful towards them. We knew they have a fantastic team and good strikers," he said.
"We were defensively better than we were attacking. We could have scored more goals but the team played well."