Fulham boss Mark Hughes explained that the club had opted for surgery after treatment had not cured the problem.
Hit for six: Duff (left) is set for a six week spell on the sidelines after having Achilles surgery
Hughes said: 'It just got to the point where one course of treatment was compromising another course of treatment.
'We made a decision that the best route was to get it sorted.'
On the mend: Dempsey (centre) set to return for the Cottagers against Liverpool
Hughes reported better news regarding top-scorer - and Fulham's new record Premier League scorer - Clint Dempsey, who missed Saturday's 3-0 Barclays Premier League win at Sunderland.
'Clint's trained after the weekend. He was a bit stiff in his hamstrings on the Friday prior to the Sunderland game, so there was no point in risking him.
'I took the view to leave him behind and he's benefited from that and he should be okay.
'He's trained today and he'll train right up to the game so he should be an option for us.'
Brede Hangeland will also be available again after missing last weekend's game through illness, but Zoltan Gera and Rafik Halliche remain sidelined.
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