Rooney has struggled so far this season and his last Premier League goal from open play came in March.
“Wayne has not started this season very well but he is still a fantastic player and I know that very soon he will be back to show that,” Ronaldo told The Sun.
“He has made his decision to stay with United and sign a new contract and we must all respect that. It’s not my position to give him advice on that. But I know that things can change very quickly indeed.
“I didn’t start this season very well, either. But it is difficult to play at the highest level all the time. I just stayed calm, worked hard and did my job for the team and suddenly I scored a few goals and everything changed.
“Football is like that. Sometimes we play bad and then you score a goal and everyone thinks differently about you. That will happen for Wayne.”