Early reports suggested two to three weeks after pulling his right hamstring in the Champions League victory over Marseille.
But Benitez told Liverpoolfc.tv: "You cannot guarantee when he'll be back. I was with him and the physios two minutes ago and he's already working. But it always depends on the player.
"It's a blow especially because we lost him before and we've been talking about the understanding between him and Robbie Keane, and him and Steven Gerrard.
"It's important not to put too much pressure on it and for him to keep working."
Benitez met with Torres and the club's medical staff Friday morning to assess what can be done about the Spain international's reoccurring injury.
"Clearly because he has been injured before it's a concern," added the boss. "We are working with the medical staff trying to analyse the reason why and trying to prevent it happening again.
"He was sharp, so it was a surprise. It was the end of the game. We just need to do exercises and check the balance of the player."