He feels some opposition defenders are targeting Ronaldo as a way of stopping him from playing the game.
"Too many defenders are pursuing a policy of stopping Ronaldo by fair means of foul," Ferguson told The Sun.
"Some tackles on him this season have been ridiculous.
"I don't mean defenders should let him through, but I do expect refs to protect him more.
"Players like Ronaldo, whom fans want to see, need to be encouraged, not kicked out of the game."