Earls sustained a pectoral muscle injury in the closing stages of the chastening 42-10 defeat in the series opener against the All Blacks and has been told he will play no part in Saturday's rematch.
He will not return home, though, and will be monitored ahead of the third Test.
Earls played at inside centre against the All Blacks but the versatile back was considered as Rob Kearney's understudy at full-back too.
His injury means more cover is needed in that area, hence Duffy's summons.
Ireland team manager Mick Kearney said: "Gavin will arrive in Christchurch on Tuesday and we'll meet up with him when we arrive on Wednesday.
"He was under consideration and he was on the stand-by list but we felt that Keith has played a fair bit of rugby at full-back so we were adequately covered there."
Kearney added: "Keith has a tear of one of his pectoral muscles. It showed up during an MRI scan yesterday. He is unavailable for the game at the weekend and his progress will be monitored over the coming days."
Ireland also have two walking wounded in their front row, with Cian Healy and Declan Fitzpatrick also suffering injuries at Eden Park.
Healy is a major doubt for the next match, with Fitzpatrick in better shape.
"Cian took a blow to his right shoulder and he has a significant soft tissue injury there," said Kearney.
"He will be modified training this week with a view to having him right for Saturday. At the moment I would say his chances are 50/50.
"Declan Fitzpatrick has a contusion to his gluteal muscle. The hamstring is okay. He is a concern but the outlook is pretty good for the weekend."
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