Zamora underwent surgery on Sunday to correct the broken fibula sustained in last weekend's 2-1 victory over Wolves, but additional ankle ligament damage was also discovered.
It was initially thought the 29-year-old would be sidelined for three months, but he must now wait until February at the earliest for his return.
Agony: Zamora was in obvious pain when he went to ground against Wolves
Cottagers boss Mark Hughes said: 'Bobby's had his operation. He will now be in bed with his leg elevated for two weeks.
'The operation showed ligament damage so we're probably now looking at five months.
'We want to help Bobby get back quickly and there are no concerns over his ability to get back to the level he was before he got injured.'
Zamora suffered the injury midway through the first half of Saturday's win against Mick McCarthy's side.
He thumped the ground in pain and was administered oxygen before being taken to hospital where it was confirmed that he had suffered a fracture four centimetres above his right ankle.
It was cruel timing as Zamora had only made his England debut the previous weekend with further caps seemingly imminent.
With Andy Johnson still several weeks away from a first-team return Fulham must now rely on impressive new signing Moussa Dembele to spearhead their attack.
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