Ronaldo looks certain to be let loose on title rivals Arsenal on Sunday after being rested in midweek when United reached the semi-finals of Champions League at Roma's expense.
And Ferguson is delighted with the player's attitude.
He said: "Success can affect people, no question of that. We see it time and time again.
"But I do not see any signs of that in Ronaldo. The boy is intelligent and sensible enough to realise what football is about.
"The great quality is to keep your ability - and your feet on the ground. Yes, he will go and express himself in a way that is extravagant at times.
"But that is a measure of his courage and real ability. We will never stop him doing that.
"We want players like him to perform on the stage at Old Trafford and express themselves."
