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.
“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."