Cristiano Ronaldo warned that Manchester United require a total rebuild after Sir Jim Ratcliffe's investment in the club, and he addressed his acrimonious exit.

With Ratcliffe having acquired a 27.7% stake of United earlier this year, Ronaldo is upbeat about the team's future but warns that returning to the top tier of club football will require patience.

"They [United] need to rebuild everything, in my opinion," Ronaldo, 39, said.

"This 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 ... They're investing in the training ground ... So I'm happy because [things are changing]. I believe that the future will be bright.

"They have to rebuild from the bottom. If not, they cannot compete. It will be impossible. They can do good things, yes, they can win Carabao, yeah, but I mean, Champions League or Europa League or Premier League -- difficult, in my opinion. And I wish that I'm wrong, I wish, but [it] will be difficult."

Ronaldo moved to Saudi Arabia after the exit and has played a significant role with Al Nassr, though he has yet to lift any titles in the Gulf nation.

"I'm not happy the way it all [happened], but in the same way, we cannot control some points of our life sometimes," Ronaldo said.

"To prove that I'm right or wrong, this is not my issue anymore."

Ronaldo parted company with the Manchester club by mutual consent following an explosive interview with Piers Morgan in which he criticised the club for failing to invest in the 13 years since he first left Old Trafford.

He said does not regret having rejoined United from Juventus in 2021 even though it didn't work out, and said he only hopes for "the best" despite his unceremonious departure in November 2022.

"I said what I have to say and for me it's done," Ronaldo said. "But to be honest with you, what I wish for Manchester [United], it's what I wish for me. The best they can make, again the best team they can [have] and I love that club, you know, I'm not that kind of guy that forgot the past."

"When I'm back there, I was very happy," Ronaldo added. "I was one of the best scorers. I did unbelievable things with the club ... I did amazing."

Ronaldo, who scored a total of 145 goals in 346 appearances for United across his two stints, admitted he struggled with manager Erik ten Hag's mentality.

"Manchester United coach, you cannot say that you're not going to fight to win the League or Champions League," Ronaldo said. "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."

Ronaldo has also welcomed the appointment of his former United teammate Ruud van Nistelrooy as Ten Hag's assistant manager.

"I believe that Ruud is going to help because he was inside the club," Ronaldo said. "He knows the club. He knows the fans. If the coach listens to him, I think they can improve a little bit the club ... The people who understand football, 99% [of the time] is people who were there in the dressing room, who were players."

United have won one and lost two of their opening three Premier League games so far this season.

United are 1-2 in Premier League this season.