The former United striker, who is now Derby County manager, was asked about the possibility of the transfer in his news conference and replied: "I don't know what to make of it, to be honest.
"If you're asking me if I think it's possible then I have major doubts that it is a possibility. Cristiano has got a great legacy at United. I don't think he's going to move somewhere for financial reasons.
"I'd be unsure if that's going to happen. I think there's more chance of it being Paris Saint-Germain if he is to leave Juventus.
"On the other hand, it's football, it can happen, it has happened before. Big players and legends from Manchester United in terms of Peter Schmeichel went to City, Andy Cole went to City, Mark Hughes managed City.
"It has happened before but I think someone of Cristiano's stature I can't see it happening."
Rooney played alongside Ronaldo at Old Trafford for five years and both players helped the club win three Premier League titles and the Champions League.
Speaking in a separate interview with UK radio station TalkSPORT, Rooney added: "I think Cristiano has got a very good legacy at Manchester United and I know how proud he is as a player and a person. I can't see it, but it's football so you never know.
"I think you need to do what's right for you as a person, for your family. Everyone knows I could never play for Liverpool or Manchester City. That's obvious but everyone's different."