"Having Heskey out is a big blow because in the six games he played, he scored three goals and created some chances as well. His work rate was phenomenal," added Houllier.
"Emile has the operation last night and it was successful but he will need another three or four weeks."
Houllier confirmed that Nathan Delfouneso will make his first Premier League start after 14 appearances as a substitute during the past three seasons.
He said: "It is an ideal opportunity for the young Nathan to step up. He will definitely be the main striker.
"Young players don't stay young all their lives. At some stage they will grow and get mature. But their career usually starts when there is an opportunity.
"I know Nathan is not a newcomer because he has already played under Martin O'Neill so he knows the Premier League.
"But to him maybe it is an ideal opportunity to put his footprint on things."
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