Heynckes led Real to glory in the 1998 Champions League and spent eight years coaching in Spain.
"Of course I know Real Madrid very well, but I have got to say they are even more dangerous and effective this season," he said.
"Spanish football is currently leading the way at the moment. Barcelona are defining a new era.
"It is going to be an interesting match tomorrow between two sides with a great tradition - between two of the best sides in Europe."