Torres ruled out for six weeks
Captain Steven Gerrard and fellow attacking midfielder Yossi Benayoun also face spells on the sidelines after picking up injuries in the humiliating FA Cup third-round replay defeat by Championship Reading on Wednesday.
"Steven has a hamstring strain and will be out for a fortnight while Fernando has torn a cartilage in his right knee. The injury will require surgery and he is expected to be sidelined for six weeks," Benitez told the club's website.
"Yossi Benayoun also suffered a fractured rib in the game and will be unavailable for between three and four weeks."
The injured trio have scored more than 20 goals between them this season and their absence is another serious blow to Liverpool's faltering campaign.
Liverpool begin their Europa League bid against Romanian side Unirea Urziceni on February 18 having failed to progress beyond the Champions League group stages.
Benitez is likely to turn to Frenchman David Ngog as a replacement for Torres with new signing Maxi Rodriguez or young Spaniard Daniel Pacheco among his other options.
The Merseyside club lie seventh in the league table with 33 points from 20 games, 12 behind leaders Chelsea and five adrift of Manchester City in the coveted fourth spot.
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