Dunne is set to lock horns with Ronaldo on Sunday as City head to Old Trafford in a bid to derail their arch rivals' bid for the Premier League title.
But, while Dunne rates the Portuguese as one of the world's best players, the defender is wary of being penalised when he faces Ronaldo, who has gained a reputation for going to ground too easily.
Dunne said: "At times he can be running so fast any nick sends him tumbling. He makes the most of the situation, I suppose.
Chelsea"Some referees give free-kicks but some don't. He is the best player in the world and he plays the game to his advantage.
"Diving or not diving, he's impossible to mark when he's on form."
Dunne added that so-called simulation was on the increase in what could be seen as a criticism of Chelsea, whose UEFA Champions League semi-final against Barcelona was decided by an injury-time away goal at Stamford Bridge in midweek.
He added: "There are a lot more divers these days. It has crept in more and more but you just think 'what goes around comes around'. You dive around and teams score against you in injury-time."