Ronaldo's second spell at United ended in a high-profile break-up in November 2022 after he made stinging comments about his Dutch boss.
The 39-year-old Al-Nassr player again hit out at Ten Hag this week, saying his attitude was too negative for a club of the stature of United.
Ronaldo told the Rio Ferdinand Presents podcast: "Manchester United coach, you cannot say that you're not going to fight to win the (Premier) League or Champions League.
"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."
He also said the club need to "rebuild everything" and that it "will be difficult" for them to challenge for the biggest trophies.
Ferdinand was a teammate of Ronaldo's during his trophy-laden first spell at United, from 2003 to 2009.
Ten Hag has said he is "quite confident" of claiming more silverware this campaign after lifting the League Cup and FA Cup over the past two seasons.
But he said in July his team are a "long way away" from being ready to win the Premier League, with United not crowned champions of England since Alex Ferguson's final season in charge in 2013.
When Ronaldo's comments about his mentality were put to him at a Thursday press conference, Ten Hag said: "He said that Manchester United can't win the Premier League. No, he said this if you read the article very well.
"So, he's far away in Saudi, far from Manchester. Everyone is entitled to have an opinion. It's OK."
Ronaldo's interview came after an international break that immediately followed United's humiliating 3-0 loss at home to bitter rivals Liverpool.
But Ten Hag said he did not let it affect him.
"It doesn't impact me," he insisted. "I know in the process where we are, what we have to do, where we are going.
"I have said before we are still in a transition period. We have to integrate a lot of young players also in the team.
"We are still dealing with injuries, we have to bring the injuries back in the team. Before anyone thinks about excuses, no, we have to win every game.
"I know that, the team knows this and it doesn't matter who is available. We will focus on every game and have in every game the mindset that we have to win."
United upset Manchester City in the FA Cup final in May but they endured their worst Premier League campaign last term, finishing eighth.
The 20-time English champions, who travel to Southampton on Saturday, have lost two of their opening three league matches this season.
Luke Shaw and Rasmus Hojlund remain sidelined but new signing Manuel Ugarte could make his debut after playing for Uruguay in midweek.
