The Real Madrid ace could be returning to United in the Champions League this season.
Ferguson said: "There is nothing between the two of them, they are both the best players in the world.
"There will always be great players. You can go back to the 60s with Alfredo Di Stefano, Ferenc Puskas and Eusebio.
"Going into the 70s you had Johan Cruyff and Franz Beckenbauer and then in the 80s Marco van Basten.
"I think what we are seeing at the moment is that Messi is an exceptional player from this decade, along with Cristiano.
"I don't think the rivalry against each other bothers them.
"I think they have their own personal pride in terms of wanting to be the best and I certainly know that about Cristiano because he played here at United for six years."