The German winger suffered damage to the anterior cruciate ligament in his right knee in the Community Shield victory over Liverpool last weekend.
Guardiola admitted the full extent of the injury is to be confirmed but anticipates the midfielder will be absent until February.
GALLERY: Scott Carson | Career so far
“I don’t know,” he admitted. “Normally, with this kind of injury – it’s six or seven months.
“Hopefully, in February or March, he will come back with us.
“He played all of pre-season and in the Community Shield Final, he played.
“It’s incredibly bad news. In three pre-seasons, we have had three ACLs.
“He is a young player and hopefully, he will recover well.
“He’s in the best hands possible. We will see next week.
“All the people here will try and help him be comfortable.
“For one or two months, he will feel alone. We will be with him as much as possible.
“We will let him know we are here and we’ll be with him.”
“Not Cancelo – not yet,” he said. “He arrived two days ago.
“Everyday, I’m more convinced this deal will be one of the best this Club has done in the last few years. He’ll be an incredible midfielder.
“I say many times: he gives us something we didn’t have. Without him, it would have been so difficult to achieve – we need competition and for Kyle Walker, it’s important to have.
“Cancelo can play on the left as well but it’s good for the team. The guy who is more fit and confident will play but right now Kyle is the first one.”
With the departure of Vincent Kompany over the summer, the opportunity for a new City captain arose.
“The captain has to be what he is. He knows the Club, the Premier League… he has big respect.
“(To be a good captain), he has to be himself. There will be no problems.
“Normally, the captain is the life of the locker room – they know each other; they have fun a lot when they are together.
“There will be no problem. He will be a good captain.”
He added: “I spoke I think once with him and said: ‘I hope you can stay because you’re a player that is difficult to find in Europe in terms of personality.’
“All I can say is: ‘thank you’ for him and the Club to keep him with us for the next four years.”
Previewing Saturday’s Premier League opener against West Ham, Guardiola stated his support for former City boss Manuel Pellegrini.
“Last season, they finished quite well,” he continued.
“His biggest quality is identifying how his team plays.
“The start was tough but I know he is a calm person and the West Ham board support him.
“At the end of the season, they had a good result at Tottenham away and against Watford so his teams have the quality.
“Up front, they have (Manuel) Lanzini and many good players.
“We were better in some periods (in previous fixtures) but it’s in the past. Last time, we won 4-0 but it was a fake result – we didn’t create too many but were so clinical up front.
“It’s the first game of the season, we played important game in the Community Shield but I saw the team quite well this week – all incredibly good in training.
“It’s the first game; anything can happen. Away, it is always tricky but we are ready and we have trained well this week.”