The Portuguese legend had two spells at Old Trafford and thinks that Sir Jim Ratcliffe and his INEOS team need to make radical changes in order to get the Red Devils competing for the Premier League title again.
Ronaldo told the Rio Ferdinand Presents podcast: "They need to rebuild everything, in my opinion.
"The club needs time to rebuild because it's still one of the best clubs in the world, but they need to change. They understand that this is the only way.
"This is why they show, they start to change again, the structure of the club, the infrastructures and everything. The owners of the club are investing in the training ground so I'm happy [because things are changing].
"I believe that the future will be bright. I believe, but they don't depend only on the talents. They have to rebuild from the bottom. If not, they cannot compete. It will be impossible."
Ronaldo left United under a cloud in 2022 after he criticised manager Erik ten Hag in a TV interview with Piers Morgan and he aimed a fresh barb at the coach for suggesting that United weren't in a position to compete for the Premier League and Champions League.
He said: "The coach says they cannot compete to win the [Premier] League and Champions League.
"As Manchester United coach, you cannot say that you're not going to fight to win the league or Champions League.
"Mentally [you can] say maybe we don't have that potential, but I cannot say that. We're going to try, you have to try.
"What I wish for Manchester [United], it's what I wish for me - [to be] the best team they can.
"I love that club... I'm not that kind of guy that forgot the past."