The 25-year-old Spain-born flanker injured the cruciate ligament in his left knee.
The 25-year-old Spain-born flanker -- who scored the first try in their 40-29 win over the All Blacks in Chicago on Saturday to end a 28 Test and 111 year hoodoo -- injured the cruciate ligament in his left knee in the first-half.
The most optimistic prognosis of a return to action is six months with the worst case scenario nine months.
Ireland coach Joe Schmidt has added six players to the original squad he named a fortnight ago as they build towards playing Canada on Saturday at Lansdowne Road.
They follow it with a second Test against the All Blacks on November 19th and round off their programme with a match against 2015 World Cup finalists Australia on the 26th.
Niyi Adeolukun (Connacht), Finlay Bealham (Connacht), Rory Best (Ulster/capt), Joey Carbery (Leinster), Sean Cronin (Leinster), Ultan Dillane (Connacht), Keith Earls (Munster), Tadgh Furlong (Leinster), Craig Gilroy (Ulster), Cian Healy (Leinster), Jamie Heaslip (Leinster), Iain Henderson (Ulster), Robbie Henshaw (Leinster), Billy Holland (Munster), Paddy Jackson (Ulster), Rob Kearney (Leinster), Dave Kilcoyne (Munster), Dan Leavy (Leinster), Kieran Marmion (Connacht), Luke Marshall (Ulster), Jack McGrath (Leinster), Luke McGrath (Leinster), Conor Murray (Munster), Sean O'Brien (Leinster), Jack O'Donoghue (Munster), Tiernan O'Halloran (Connacht), Peter O'Mahony (Munster), Jared Payne (Ulster), Garry Ringrose (Leinster), Donnacha Ryan (Munster), John Ryan (Munster), Jonathan Sexton (Leinster), CJ Stander (Munster), Darren Sweetnam (Munster), Devin Toner (Leinster), James Tracy (Leinster), Andrew Trimble (Ulster), Josh van der Flier (Leinster), Simon Zebo (Munster)