Ronaldo's relationship with Ten Hag deteriorated rapidly during the Dutchman's first few months with the Red Devils back in 2022.
The player made 16 appearances for United under Ten Hag, scoring just three goals in what was an intense time at the club.
After missing the club's pre-season tour while his desire to leave was widely reported, he refused to come on as a substitute against Spurs and walked down the tunnel before full-time.
Ten Hag dropped him for the next game, and a month later Ronaldo sat down with Piers Morgan, where he said he felt "betrayed" by Ten Hag due to a lack of respect.
"He let the team down, and he let me down, because we planned with him," Ten Hag said in an interview with The Athletic about his time at United.
"I flew out to Portugal to speak with him about what I was aiming for before I came to United. Obviously, he’s one of the best in football history. I respected him a lot. But at that time, as a manager, I had to make a rational decision and keep the players I admire in."
United announced Ronaldo's departure with immediate effect as the interview made the situation untenable, according to reports.
Ten Hag suggested that the legendary forward did not apply himself in training, which may have led to his poor form and eventual departure.
"I confronted him with this," said Ten Hag, who is now the technical director at FC Twente.
"Many times I say, 'Ronnie, everyone wants to work for you to bring you in the right position, but if you can't bring yourself in the box, you can't score.'
"He was not capable to do this. It's the wrong idea to put this to me that I expected him to do the pressing, that he's exhausted. But he couldn't. He ran neither in possession nor out of possession.
"The reason was because he didn't prepare himself."
"He was telling the players, 'Save your legs at the training'. I understand it when you are top fit; you have sessions, you can save the legs. But first you have to train yourself to get into the right fitness levels," the Dutchman added.
Ronaldo has since stormed out of Portugal’s camp during the last international break amid a feud with manager Jorge Jesus, as he continues to create headlines at 41 years old.