Ronaldo was 36 when he returned to Manchester United from Juventus in 2021, when Ole Gunnar Solskjaer was manager.
Under Ten Hag, he scored just three goals in 16 outings before his exit during the 2022-23 campaign.
"It's the wrong idea to put this to me that I expected him to do the pressing, that he's exhausted," added Ten Hag. "But he couldn't. He ran neither in possession nor out of possession. The reason was because he didn't prepare himself.
"Every top team has to embrace the top player, but the top player has to work for the team.
"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 publication of Ten Hag's interview comes as Ronaldo has been given a 30-day suspension and fined 255 euros (£215) by the Portuguese Football Federation for leaving his country's international camp last month.
Ten Hag was sacked by Manchester United in October 2024, having won the Carabao Cup in 2023 and FA Cup in 2024.
On his time at Old Trafford, Ten Hag added he wanted to sign Frenkie de Jong, Harry Kane and Declan Rice, and was against the sale of Scott McTominay to Napoli in August 2024.
"I said for the first season, 'centre-half, there's a problem, centre-midfield, there is a problem, and especially I need a strategist, then we need goals'. And that was the biggest issue in all my three years: goals," said Ten Hag.