The Al Nassr star made the comments at the Globe Soccer Awards in Dubai.
Asked about City's Treble, Ronaldo said: "Manchester City did amazing. Congratulations on their season. Players, coach. Superb."
The 38-year-old added: "Finally they win the Champions League, they probably deserved it two years ago. I enjoy seeing them playing."
Ronaldo was linked with City when leaving Juventus before eventually signing for United for a second time.