Ronaldo, who left United in the summer for Real Madrid, believes that Rooney would be a big hit in Spain.
"Wayne would be a sensation out here. I think we could do with Rooney to help us out. He would be brilliant if he came," the Portuguese winger has told the Sun newspaper.
"This is the ideal place for the world's best players and there's little doubt he is one of them.
"Wayne has all the skill and determination you need to make a fine career here. He has shown just how good he is over the last few years in England.
"There's no doubt he would be the same sort of player in Spain. It would be lovely to see him here one day at Real Madrid.
"I realise it would take a lot for him to move from England. He loves the country, he loves Manchester and he is a Liverpool boy.
"United is a good club for him, but you never know what could happen in the future."