Petrov's prognosis is at the higher end of the timescale set last week by manager Gerard Houllier, who confirmed the midfielder would be sidelined until at least Christmas.
Petrov suffered a partial medial knee ligament tear in last month's 1-0 Premier League defeat at Sunderland.
He said on his club's official website: "I'm bitterly disappointed not to be involved.
"I feel okay, but the knee injury I picked up against Sunderland means I will be out for around three months.
"It was one of those things and you have just got to accept that sometimes these things happen in football.
"It is the first time I have been out for such a lengthy period since I suffered a broken leg when playing for Celtic 10 years ago.
"But I have every confidence in the boys to go out there and perform to the best of their ability.
"The team have been working really hard under the new manager and we have been playing well recently.
"We've been creating a lot of chances too so hopefully we can start putting a few more of them away.
"But it will take time and that is something that every team needs under a new manager."
Nigel Reo-Coker has taken over the armband in Petrov's absence and he has been backed by team-mate Barry Bannan to do justice to the role.
"Nigel has done really well," Bannan said. "He is an experienced player. He knows what he is doing.
"It is not as if he is new to it because he captained West Ham and Wimbledon when he was younger.
"So, he is a great replacement captain because he has been there before and done it before."