Sane flew to Germany on Wednesday ahead of a proposed €49 million move to Bayern Munich. He is set to sign a five-year contract with the Bundesliga champions and will not play for City again.
"No, no. no, he decided to leave," Guardiola told a news conference on Wednesday. "Why should he decide to come back? When he decided to move to Dortmund it's not because he is going to decide to come back later. It would make no sense. He didn't want to be here, so that is why he doesn't want to come back here."
After surrendering the Premier League title to Liverpool, Guardiola is facing the prospect of rebuilding his squad ahead of City's attempts to win it back next season. Sane has already headed for the exit and he will be followed by David Silva, who will leave at the end of the campaign.
"After we finish we reflect," said Guardiola. "I want to sit with the club, take a little time to think with the club what we have to do, which players should we replace -- Leroy and the others -- we have to see what happens," said Guardiola. "Some players have to be replaced. But it's not a disaster thing, or bad thing, it's part of football for the age, for the normal situation and circumstances.
"It happens in other clubs and players who are considered important players for the club decide to leave. They don't want to be here and that was the case for Jadon Sancho and Leroy, especially Leroy because Jadon Sancho was in the academy.
"It happens at clubs around the world, you never can be eternal, and we have to accept it and try to take the right decisions as a club to try to maintain the level that we have."
City go head-to-head with newly crowned champions Liverpool behind closed doors at the Etihad Stadium on Thursday. Guardiola says his players will give Jurgen Klopp's side a guard of honour and midfielder Ilkay Gundogan insists it is deserved.
"I am not sure if I have done it before to be honest," said Gundogan. "It is a tradition in England so obviously we will follow tradition. They deserve it because they are the champions. I can imagine that people from the outside such as media and fans make it bigger than it actually is.
"If someone is better than you, whether that is in a game or competition, you have to accept it and congratulate them. Next time, when you get the chance, you have to take the chance and make it better, that is how I see it."