Veteran manager has branded the Portuguese ace, who made his name at Old Trafford, as the best in the business, but believes his Argentine rival is also a "fantastic" footballer
The two teams are set to clash for the third time in the space of a fortnight, this time with a place in the Champions League final being on the line.
And with the first leg to be played at the Bernabeu on Wednesday, the Scot has added his thoughts by pointing towards his former player as the best in the business.
"Well, I had Ronaldo here as a player and I think he is the best player in the world," Ferguson told the club's official website.
"He can play with both feet, he has fantastic skill, strength and bravery, and he’s a greater header of the ball. But Messi is an absolutely fantastic player also.
"It’s difficult to choose between the two, it’s the toss of a coin.
"It depends on how you view your player. The fact that I had Ronaldo here so long, I know him well. I saw how he dedicated himself in training sessions to becoming the best player in the world and so that would weigh heavily on my view of him as a player and as a person.
"But you look at the boy Messi and you know that he’s a serious player. He loves playing football. He’s as brave as a lion because he will always take the ball in any situation. They are different types of players but there’s no doubt both provide a fantastic end product."