Liverpool will be without influential midfielder Mohamed Sissoko for three months as he requires surgery on his dislocated shoulder.
Sissoko was injured in the first half at Birmingham
Sissoko sustained the injury during the 1-0 Carling Cup win at Birmingham on Wednesday after landing awkwardly following a challenge on Medhi Nafti.
"Unfortunately Momo requires surgery which is scheduled to take place next week," said a Liverpool spokesman.
"It is anticipated he will be out of action for three months."
Sissoko was sidelined last season when he suffered a serious eye injury during Liverpool's 1-0 defeat by Benfica in the Champions League.
And Benitez added: "They are not very common injuries, the kind he has sustained, it was his eye last season and now the shoulder.
"It was an accident but it was clear something was wrong. Momo is a player who doesn't like to stay on the ground.
"Now we have to manage with the players we have until January and I am happy with the squad but when you lose a player that is playing every game like Momo, it is a problem."
Sissoko received treatment for more than five minutes before he was carried off from the field on a stretcher and replaced by Xavi Alonso.
Benitez signed Sissoko for £5m from Spanish club Valencia in July 2005.