Ferguson claims the winger is one of the most fouled players in the top-flight - and that is backed up by official Premier League statistics - but that the 23-year-old will not allow himself to be kicked out of the game.
The United manager said: "(Arsenal manager) Arsene Wenger recently complained that his team are on the receiving end of more fouls than any other.
"I'm not sure about that but what I do know is that Cristiano Ronaldo must be one of the most fouled players in the league.
"The fouls are frustrating, especially for Ronaldo, and I have nothing but admiration for the way he picks himself up and keeps running at people.
"Time after time he is tackled unfairly and often opponents try to indicate that he has dived.
"Of course, when Ronaldo is running at full speed sometimes it doesn't take much to knock him over but the truth is that too many opponents don't know how to tackle properly. And to disguise their shortcomings they accuse him of falling deliberately.
"I am pleased for him with the punishment he has taken lately. I have been making the point about his courage and the fact he is clearly determined that he is not to be kicked out of the game."
According to the Premier League's statistics, the Portuguese winger is in the top five most-fouled players with 61 offences against him so far this season.
Ferguson added: "I will continue to hammer away at the need for protection for football's most skilful players."