The Bulgaria international suffered the setback during the first half of Saturday's 1-0 defeat by Sunderland and is to have a scan to verify the extent of the damage but will definitely miss the midweek Carling Cup tie with Burnley and the derby clash with Birmingham at Villa Park next Sunday.
Houllier said: "He will have a scan. I think it could be a lateral ligament injury which may mean two or three weeks (out of action) if it is a grade one injury. If it's a grade two or three, maybe it could be longer."
Petrov may have been rested in any case for the Carling Cup match against Championship opposition.
But the experience and know-how of the former Celtic player will be missed in the Villa Park derby with Alex McLeish's side.