The 31-year-old's form has been questioned at times this season despite his double against Athletic Bilbao last weekend taking his goal tally to 32 in just 30 matches.
Nainggolan believes criticism of Ronaldo is testament to the astonishing standards he has set in recent years, but insists he remains one of the world's finest.
"The only thing that I see is that he scores goals every week. If they don't want him, I'd love him to come to Roma," he told Marca when asked if had seen a decline in the Portugal captain's performances.
"It's normal that these great players are questioned. They are global stars and they have themselves to blame for being so good.
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"Cristiano and [Lionel] Messi generate such expectation around the world that, once they're down just a little, people get worried unfairly.
"They're one of the best teams in the world. You can never doubt that."
Ronaldo and Madrid travel to the Stadio Olimpico to take on Roma on Wednesday in the first leg of their Champions League last-16 tie, and Nainggolan concedes Luciano Spalletti's side need a "miracle" if they are to progress to the quarter-finals.
"Facing Real Madrid will help us to know where we are in these moments, where we find ourselves in respect to these teams," he added.
"The Olimpico will help us to be better. We'll try to cause some distress for Real Madrid.
"We play at home and we have to get a good result for the return game. Okay, we'll try to pull off a miracle, perhaps I should say."