Ferguson said that the Portuguese star, whom he bought in 2000 for 12 million pounds when Ronaldo was still relatively unknown, has flourished in his current club Real Madrid and added that he never thought that Ronaldo would reach the top levels, the Mirror reports.
Hailing Ronaldo and Lionel Messi as two of the best players in the world, Ferguson said that Ronaldo is having a phenomenal rise to his career in view of his 182 goals from 179 games in Spain.
Ferguson claimed that Ronaldo, who had been sold to Madrid for a world record fee of 80 million pounds, is at the peak of his career which will remain the same for the next three years.
However, Ferguson said that despite his admiration for Ronaldo, he believes that United's Wayne Rooney and Robin van Persie can give a good challenge to the Portuguese star.
United will rely heavily on Rooney's peerless work ethic for the game against Madrid, with the forward expected to be deployed in a deeper-lying role - possibly even out wide - to help halt Real's attacking waves.
Van Persie's 23 goals this season are conclusive proof of the remarkable impact he has had at United since his 24million move from Arsenal. (ANI )