Ronaldo, who won eight trophies at United, told Piers Morgan in an interview this week that United were “not in a good path” and warned that Amorim was “not going to do miracles” at Old Trafford.
“We say in Portugal, ‘miracles only happen in Fatima,’” the 40-year-old Al Nassr forward said, adding that the Manchester club lacked long-term vision.
Amorim, who took over from Erik ten Hag in November 2024, hit back ahead of Saturday's Premier League clash at Tottenham Hotspur.
“Of course, he has a huge impact in everything he said," Amorim said on Thursday. "What we need to focus (on) is in the future. We know that we as a club made a lot of mistakes in the past, but we are trying to change that. So let's not focus on what happened. Let's focus (on) what we are doing now.
"We are changing a lot of things in the structure, the way we do things, the way we want the players to behave. We are doing that and we are improving."
Ronaldo expressed sadness over the club’s decline but insisted the problems go beyond the manager.
“You have to work with smart people to create a base for the future,” he said. "Right now, they don't have a structure."
United have shown signs of improvement, climbing to eighth, two points off second spot, after a four-game unbeaten run.
Provisional second place in the table is up for grabs on Saturday in a grudge game for Amorim's side after Spurs beat them 1-0 in last season's Europa League final to prevent the Red Devils from playing in the Champions League this season.
Amorim credited internal reforms for the turnaround.
"Characteristics of the players are different in the team now," he said. "We also have players who played last year. But we are a better team. We understand the game better. We are more confident.
"We reached the Europa League final confident that we could win it. But in this moment we're playing with a different confidence."
Amorim added that there was a silver lining to missing out on European football this season.
"It's us using the positives," he said. "Any situation you have in life, you can look at it positively or in a bad way.
"If you ask me whether I prefer to be here with the (Europa League) and Champions League football, I might say yes.
"But on the other side, you have more time to train and know your players. Arrange everything and prepare with the staff and players. So we try to use the positive thing of not playing in the midweek games."
