Houllier is hopeful Petrov will be back inside two months from the lateral knee ligament injury suffered against Sunderland on Saturday.
Houllier said: "The worse case is that Stiliyan will be out for a minimum of two months. I think we should know by the end of the week whether he needs an operation or not. Let's keep our fingers crossed he doesn't require one as it could then be treated conservatively which will be the best scenario."
He added: "The worst scenario, of course, is needing the operation.
"But Stiliyan is a strong lad who has had to go through injuries before and he has always come back."
Houllier is optimistic full-back Luke Young will be able to resume training next week after being sidelined with a hamstring injury.
But striker Gabriel Agbonlahor is not yet ready to start his comeback after undergoing groin surgery three weeks ago.
Houllier said: "Luke Young is slowly gradually coming back to normal.
"Next week he should be able to train with the others.
"Regarding Gabby, it will be one or two weeks before he joins the group (for training)."