Ronaldo's presence looms heavy over the Champions League tie, with the United boss admitting the player he spent six years nurturing at Old Trafford is now better than ever.
"Absolutely, he is now at the very peak of his career," said Ferguson.
"He was still young man when he left us and has continued to flourish, but I never thought he would get to that level.
"He and Messi are the best players in the world. To score the goals he has is phenomenal."