He said: "Some tackles on our man have been ridiculous this season. I don't mean that defenders should let him through. But I do expect refs to give him much more protection.
"Officials should be celebrating the arrival of a great player and give him the protection he is entitled to. Special players need to be encouraged rather than kicked out of the game.
"I am increasingly worried that too many defenders now have a policy of stopping him by fair means or foul."
Ronaldo served a three-match ban in August after reacting to a foul by Portsmouth's Richard Hughes with an apparent head-butt.
Ferguson said: "Ronaldo was fouled all night but then got sent off.
"Considering the stick he takes, for the most part he keeps cool."