The Portugal star, who now plays for Al Nassr in Saudi Arabia, fell out with Ten Hag before his second spell at Old Trafford ended late in 2022.
Ronaldo believes his former club needs sweeping changes but that Ten Hag should not rule out winning major trophies in the meantime.
The Dutchman has said he is “quite confident” of claiming more silverware this campaign after lifting the League Cup and FA Cup over the past two seasons.
But he said in July his team are a “long way away” from being ready to win the Premier League, which they have not won since Alex Ferguson’s final season in charge in 2013.
“Manchester, they need to rebuild everything, in my opinion,” Ronaldo said in an episode of the Rio Ferdinand Presents podcast, due to be released on Thursday.
“The coach, they say they cannot compete to win the league and Champions League. Manchester United coach, you cannot say that you’re not going to fight to win the league or Champions League.
“You have to be, to mentally say, listen, maybe we don’t have that potential, but I cannot say that. We’re going to try. You have to try.”
United upset Manchester City in the FA Cup final in May but they endured their worst Premier League campaign last term, finishing eighth, and the 20-time English champions have lost two of their opening three matches this season.
“This club needs time to rebuild because it’s still one of the best clubs in the world, but they need to change,” Ronaldo said.
“They understand that. This is the only way. This is why they show, they start to change again, the structure of the club, the president, the infrastructures and everything. The owners of the club.”
‘Bright future’
British billionaire Jim Ratcliffe completed his purchase of a minority stake in the club in February and has since overseen major changes in the club hierarchy after taking control of football operations.
“I believe that the future will be bright,” said Ronaldo. “I believe, but they don’t depend only on the talents. It’s more than that. It’s all club, all, not only players, club, everything as a unit, united.
“They have to rebuild from the bottom. If not, they cannot compete. It will be impossible.”
Five-time Ballon d’Or winner Ronaldo, who spent six seasons with United earlier in his career, scored 27 goals in 54 appearances during his second spell at Old Trafford after signing from Juventus in 2021.
Following his exit just over a year later, he claimed he felt “betrayed”, while saying of Ten Hag: “I don’t have respect for him because he doesn’t show respect for me.”
However, the 39-year-old said he still loves the club.
“I’m not happy the way it all happened but, in the same way, we cannot control some points of our life sometimes, but it’s done, already done,” he said.
“To prove that I’m right or wrong, this is not my issue any more. I say what I have to say and for me it’s done.”
He added: “I love that club. I’m not that kind of guy that forgot the past.”
© Agence France-Presse
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