Erik ten Hag has told ESPN he does not believe Cristiano Ronaldo was fit or committed enough during their time together at Manchester United, adding that he feels the five-time Ballon d'Or winner could "no longer deliver" on the pitch after leaving Juventus in 2021.
Ten Hag also said the notion that one of Ronaldo's defining characteristics is his dedication to pushing his physical limits is "not entirely true, but that is the image."
Ronaldo left Juventus to return to Old Trafford where he scored 27 goals in a 54-game spell under Ole Gunnar Solskjaer and then Ten Hag.
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The former Ajax boss did not see eye to eye with the Portugal star during the latter stages of his time in charge at United as he regularly opted to leave Ronaldo out of the starting XI.
United terminated Ronaldo's contract in November 2022 after he said in an interview with Piers Morgan that he felt "betrayed" by the Dutch manager and criticised the club's hierarchy.
Ten Hag, who is now the technical director of Dutch side FC Twente, was sacked nine games into the 2023-24 season with United 14th in the Premier League table.
Reflecting on his time at the club, Ten Had told ESPN NL's Goedemorgen Eredivisie on Sunday: "The challenge is to get the team functioning, really as always. Every player is important in that and you need top players to be able to win. Every top team must embrace the top player, but the top player must work for the team. And that was simply the problem with Ronaldo."
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He continued: "The players wanted to work for him and they looked up to him because let there be no misunderstanding about that. He is of course a tremendous player, one of the greats of all time. But that is also the reality over the last seven, eight years he simply can no longer deliver. From Juventus onwards it's simply over.
"And that is actually the great gift of Real Madrid -- they always know when to say goodbye to players at the right moment. They recognise when a player's level is declining and they say goodbye at a moment when to the outside world, they still appear to be at their peak. But they can no longer actually deliver."
Ronaldo was 36 by the time he returned to the Premier League and was criticised by commentators and pundits for his perceived inability to press opposition defenders.
Rather than putting Ronaldo's physical decline down to his advancing years, Ten Hag said the former Real Madrid forward "could no longer bring himself to push through the pain."
"I naturally gave him a special status," Ten Hag said. "I also went to him, I spoke to him prior to that job, I explained to him how we were going to play, what his role in that would be, what I expected of him, and what he could expect from me and from the team. But ultimately you also have to deliver yourself and he was no longer capable of pushing himself to the limit.
"As I just said, he could no longer bring himself to push through the pain, to give everything every single day and to train at his maximum in order to get fit. And if you're not fit in the Premier League, Champions League, or with the Portuguese national team, well, then you simply have no chance."
Pushed on how that tallies with the understanding that one of Ronaldo's most notable qualities is his dedication to his physical fitness, Ten Hag replied: "That's not entirely true, but that is the image. And that is of course how he became so good. But ultimately he could no longer bring himself to do that.
"If you can't do it, well, then the laws of the Premier League are merciless. Then you can no longer keep up, you are no longer able to get into the box. He can still finish a chance, you still see that. But to get into that power and intensity of the Premier League, that's no longer possible. For that you need to be fit."
Asked if there ever a time when Ronaldo scored and he was happy with his star player, Ten Hag said: "That didn't happen because he arrived very late due to personal circumstances. By then the season had already started and things went wrong straight away. He wasn't fit and couldn't deliver."
Ten Hag was glowing in his praise of Ronaldo's overall career, saying he has "given so much to football," but said that at Old Trafford, he needed to be "clear and decisive" with Ronaldo in order to benefit the team as whole.
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Ten Hag compares Ronaldo to Messi
Ten Hag's comments come after Ronaldo was suspended for 30 days by the Portuguese Football Federation after his walkout on the national team during the October international break.
He was also fined €255 ($285.90) but he has said that he is available to play for Portugal in the future.
"No one is bigger than a club or a country," Ten Hag said. "Not even Cristiano Ronaldo and not even Messi. In the end, the team always comes first. And so, as I just said, every top team must embrace the top player, but the top player must yield to the team. And if he no longer does that, well then it can't be a success and it will never become a success. Then it simply stops.
"That is also what's remarkable about Messi -- how he worked at Argentina. That was fantastic to see. But that also required a big investment from Messi himself. Ultimately everyone went along with it and everyone worked for him but he of course gave an enormous amount back."
Ten Hag had explained his strained relationship with Ronaldo during an interview with The Athletic last month.
Asked if the terms of his United exit had forbade him from giving interviews for two years, Ten Hag told ESPN: "I was allowed to give interviews, but not to damage the club. But I wouldn't have done that anyway -- that goes without saying.
"United gave me so much and I worked there with such great pleasure, with the people. [We] simply had a fantastic relationship, built fantastic connections. And the fans, such fantastic moments in those FA Cup finals, League Cup, [they] always supported. So there's no bitterness."