The Portugal captain admitted that his side were expecting a tough game against the Bundesliga champions, although he feels that Dortmund's second goal had a touch of luck about it.
"We expected a tough game and Dortmund certainly made sure of that," said Ronaldo.
"They were a bit lucky with the second goal, but it's no reason for us to panic. We've still got several games to get the points we need.
"It's always 50-50 when two great teams play against one another and tonight we were on the losing side. Losing is part and parcel of football. Both teams have some very good attacking players, we saw that much today. I'm sure it was a great match for the fans, less so for us."
The two sides meet in the reverse fixture at the Santiago Bernabeu in two weeks' time.