At first it was unclear whether Ferris would miss just the South Africa match or recover in time for the match against Argentina but Ireland head coach Declan Kidney has conceded he is unlikely to play any part.
"He has significant ankle ligament damage and is continuing to receive intensive treatment," said Kidney.
"But at this stage, we don't expect him to be available for the game against Argentina on Saturday week."
Meanwhile, Ulster quartet Tom Court, Paul Marshall, Craig Gilroy and Roger Wilson have been added to the squad by Kidney.
Marshall is expected to enter the fray against Fiji at some stage with scrum-half Eoin Reddan suffering an ankle knock against South Africa.
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