Sept 12 (Reuters) - Cristiano Ronaldo said his former club Manchester United ( MANU ) must rebuild everything from the bottom up if they are to compete for the top titles again.

The 39-year-old Portugal forward won three Premier League titles, the Champions League and a Club World Cup crown during his first stint at United from 2003-2009 and said he still loves the club, who finished eighth in the league last season.

Speaking on the 'Rio Ferdinand Presents' podcast set to air on Thursday, Ronaldo said he was happy with the way the club's administrators, led by INEOS boss Jim Ratcliffe, were investing in infrastructure.

"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," British media quoted him as saying.

"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."

United manager Erik Ten Hag has come under pressure already after two defeats in their opening three games and Ronaldo said he should lean on his former United teammate Ruud van Nistelrooy, who was hired as an assistant coach.

"You cannot rebuild a club without knowledge," he said. "I believe that Ruud is going to help because he was inside the club. He knows the club, he knows the fans. If the coach listens to him, I think they can improve a little bit the club."

United, who are 14th in the league, visit Southampton on Saturday.