"Ronaldo has a gastritis and did not train today in order to recover and rest," Portugal coach Fernando Santos said. "It's a condition that doesn't help much, as it affects the players a lot, they lose a lot of liquid, they weaken. He won't be ready for tomorrow, for sure."
Asked whether the Portugal captain really had a stomach bug or it was just an excuse after the uproar that Ronaldo's explosive interview had generated around the world, Santos laughed and said: "If it were another player, we wouldn't question it, but yes, he really has gastritis and is in no condition to play."
In Sunday's interview, Ronaldo, 37, criticised United and said he has no respect for coach Erik ten Hag, leaving his future at Old Trafford uncertain.
He joined the national team on Monday to prepare for his fifth World Cup.
Santos said he respects Ronaldo's decision and doesn't believe it will affect his national team.
"He [Ronaldo] didn't have to inform us about the interview," Santos said. "Isn't he free [to make his decisions]?
"What I'm interested in is what is being spoken in our camp and not what is being said outside. We have to respect his decision.
"We have to respect the interview he gave. It has nothing to do with the national team.
"It was the player, the man, who decided to give an interview. He doesn't talk about the national team; it's a very personal interview, and we have to respect that. I didn't see anyone in the national team commenting on the matter.
"In the interview, he talks about his club and the relationship he has with the club. We have to respect it, as we respect the decisions that other players make. It has no impact on the national team."
Ronaldo, who is the top men's international goal scorer with 117 goals in his career, is in the midst of his most difficult campaign.
He has scored just three goals in 16 games across all competitions and started in just four Premier League games.
Asked whether he feels obliged to start Ronaldo, Santos said: "I don't feel any obligations. No one does. All the players that are here can be starters."
Ten Hag has told Manchester United bosses that Ronaldo should not play for the club again, sources have told ESPN.
United on Wednesday reportedly took down a giant mural containing Ronaldo's image at Old Trafford.
The image in question forms a large part of the exterior of the stadium.
The five-time Ballon d'Or winner is under contract with United until June 2023 and has an option to extend it for a further year.