The 29-year-old Munster front row underwent a scan on Monday and initial fears he had torn his left calf muscle proved correct.
He will miss Sunday's final autumn international against the Pacific Islands and will not return for Ireland until the RBS 6 Nations opener against Wales on February 3.
Rookie Ulster prop Bryan Young, who started ahead of Horan for the Wallaby clash, will continue at loosehead against the Islanders.
Ireland coach Eddie O'Sullivan has added 10 players to his 22-strong squad in preparation for the final rugby international at Lansdowne Road before it undergoes redevelopment in January.
Luke Fitzgerald, Eoin Reddan, Barry Murphy and Jeremy Staunton strengthen the back division while Matt McCullough, Jamie Heaslip, Simon Best, Stephen Ferris, Ronnie McCormack and Keith Gleeson have bolstered the forwards.
O'Sullivan names his team to face the Islanders on Wednesday and is expected to give his fringe players a run out against the weakest of the month's opposition.