The Red Devils striker ended a week of feverish speculation over his future by putting pen to paper on a new five-year contract at Old Trafford.
Despite committing himself to the club he joined from Everton in 2004, Rooney remains mired in a dismal run, with his last Premier League goal from open play coming way back in March.
But Ronaldo, who left United for Real Madrid in a world record £80million deal in the summer of 2009, is confident his old colleague will soon be terrorising defences once again.
"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."
And Ronaldo believes veteran manager Sir Alex Ferguson, who he rates as the best in the business alongside Real boss Jose Mourinho, can help the Red Devils overcome their sticky start to the season.
Despite being the only unbeaten side in the top flight this term, United have come under scrutiny for a number of below-par displays, including allowing leads to slip against Fulham, Everton and West Brom.
"When I was in England United won many titles, trophies and cups," Ronaldo said. "Last year they didn't win anything special and this time they don't play so good, but this is just a moment.
"We have to give United time to find themselves again. Although their style is similar, they have bought some different players and they have not always been at 100 per cent. But I am sure that they will be good again very soon.
"For five years I enjoyed working with Sir Alex Ferguson. I always had a good relationship with him and now I think of him as my friend as well as a great manager.
"For me Ferguson and Mourinho are the best managers in the world. One is young and one is older. But they have both won everything in football."