Ronaldo is back to today's clash against Everton and Ferguson said: "It is possible some players might try it on with Cristiano.
Fergie will be relieved to have Ronaldo back as United had lost their spark in his absence.
The Scot said: "It is a big plus having Cristiano back.
"You miss players who can influence games and change their pattern. He creates opportunities and scores as we saw last year so he has been a miss."
Ronaldo, 22, has been kicking his heels during his suspension and is raring to return.
He said: "I hate being suspended because I grow bored when I cannot play.
"But sometimes, in the heat of the moment on the pitch, things happen and only a professional player can understand that."