However, Ferguson feels that United's star man Van Persie is not far off reaching the same level as the Portugal international.
"Cristiano was such a young man when he left me and you can see how he has flourished in Madrid," said Ferguson. "I never thought he would get the number of goals he has scored.
"He and Lionel Messi are the best players in the world and Cristiano's goal record is phenomenal.
"Robin is now approaching that level. He has been a breath of fresh air."
The Dutchman has scored 23 goals for United since completing a £24m move from Arsenal in August.