Van Persie has netted 23 times in all competitions for the Red Devils since completing a £24m switch to Old Trafford from Arsenal last summer and currently tops the Premier League goal-scoring charts.
Ronaldo, who scored 118 goals in 292 games during his six-year stint at United, is set to line up against his former club at the Bernabéu on Wednesday.
And Ferguson admits that he has been delighted with the impact that the 29-year-old Dutchman has made since moving to the Premier League leaders.
“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.”
MORE: Latest Man Utd news
MORE: Latest Arsenal news
MORE: The latest football news