Sane suffered the injury less than 15 minutes into the Community Shield match against Liverpool at Wembley last Sunday.
The injury means that any potential sale to Bayern Munich this month is off as the 23-year-old German begins a long spell of rehabilitation with an operation in Barcelona planned for next week.
Guardiola insists that a move was never on the cards, and his job now is to look after his player.
“Normally this kind of ACL injury is six or seven months,” he said on the eve of City’s beginning of the defence of their Premier League title away at West Ham.
“Hopefully in February or March, he can be back with us.
“I did not think about him leaving. I always said he is our player and he didn’t tell me that he wanted to leave.
“I always thought he was ours and he played in the Community Shield.
“He is young and hopefully he will recover well. He is in the best hands possible in terms of the surgery next week.”
Guardiola, who on a positive note saw Ilkay Gundogan agree a three-year contract extension earlier yesterday, said they would be there to support Sane.
“He will feel alone after one, two months but we will help him to feel as comfortable as possible,” said the 48-year-old Spaniard.
“It is incredibly bad news as in three years we have had three ACLs.
“In the first season, we didn’t have Ilkay Gundogan, then Benjamin Mendy. Now Leroy.
“So let’s have him feel that we are here for him and we will wait.”
Guardiola also revealed David Silva will be City’s new captain as the Spaniard begins his tenth and final season at the club.
He replaces Vincent Kompany who left to become the player-manager at Anderlecht.
Silva has been around for all of City’s success as he has collected ten major trophies at the Etihad Stadium.
He is a very different personality to Kompany, but it was a unanimous decision by the squad to give him the armband, while Fernandinho, Kevin De Bruyne and Sergio Aguero will be vice-captains.
“David has been here a decade, he knows the club, he knows the Premier League,” said Guardiola.
“He has big respect from his team-mates. They had to take the decision that was best for the team and the club.
“David has to be himself. He has to make decisions for the team and do what he thinks is right.
“But what the captain is like depends on what the locker room is like.
“They are nice guys, they know each other, they laugh a lot so it will not be a problem.
“David will be a good captain.”
Of City’s major summer signings, Rodri is likely to make his league debut at West Ham, but Joao Cancelo will have to wait after only arriving from Juventus on Wednesday.
“Cancelo will not play but he will settle well,” said Guardiola.
“Every day I am more convinced that Rodri will be one of the best deals this club has done for many years.
“He will be incredible for us for many years.”