Scrum-half Conor Murray has been ruled out of the rest of Ireland's autumn campaign after injuring his groin in Saturday's win over world champions South Africa in Dublin.
Murray's 100th cap ended early when he was forced off in the first half.
Ulster's Iain Henderson has returned to the squad, while uncapped Leinster prop Michael Milne has been called up.
Robbie Henshaw, who missed Saturday's Test, will return to training this week ahead of Fiji's visit to Dublin.
Stuart McCloskey, Tadhg Furlong and Johnny Sexton are also recovering from injuries picked up against the Boks.
McCloskey, who replaced Henshaw in Andy Farrell's side against South Africa, is "recovering quickly" from the arm issue that forced him off before half-time.
In a squad update issued on Monday, the Irish Rugby Football Union (IRFU) said Furlong is expected to train this week despite an ankle problem.
It added that captain Sexton is "recovering well" from a dead leg.
Ciaran Frawley and Joe McCarthy, who featured in Ireland A's defeat by an All Blacks XV in Dublin on Friday, and Ryan Baird, will all miss the Fiji game.
Jack Crowley, Caolin Blade, Max Deegan and Scott Penny have all been promoted from the Ireland A side.
Ireland hope to back up Saturday's impressive defeat of the Springboks with victory over Fiji on Saturday before concluding their autumn campaign against Australia on 19 November.
Following the win over South Africa, who Ireland will meet again at next year's Rugby World Cup, Farrell dismissed the significance of Rassie Erasmus' comments from three years ago when the Springboks director of rugby branded the Irish "softies".
"I don't make a big thing about it because I don't want to get distracted," said Farrell.
"I want us to bring our own physicality because we know how to.
"I don't want to make the game too emotional because I want us to be accurate in what we do.
"I think the game has moved on a little bit now in as far as beating your chest and going out there and that'll do.
"We're trying to concentrate more and more on ourselves and I think that's showing in how we're playing at the minute."
Ireland squad
Backs: Robert Baloucoune (Ulster), Caolin Blade (Connacht), Joey Carbery (Munster), Craig Casey (Munster), Jack Crowley (Munster), Jamison Gibson-Park (Leinster), Mack Hansen (Connacht), Robbie Henshaw (Leinster), Hugo Keenan (Leinster), Michael Lowry (Ulster), Stuart McCloskey (Ulster), Calvin Nash (Munster), Jimmy O'Brien (Leinster), Garry Ringrose (Leinster), Johnny Sexton (captain, Leinster), Jacob Stockdale (Ulster).
Forwards: Finlay Bealham (Connacht), Tadhg Beirne (Munster), Jack Conan (Leinster), Max Deegan (Leinster), Caelan Doris (Leinster), Tadhg Furlong (Leinster), Dave Heffernan (Connacht), Cian Healy (Leinster), Iain Henderson (Ulster), Rob Herring (Ulster), Jeremy Loughman (Munster), Michael Milne (Leinster), Peter O'Mahony (Munster), Tom O'Toole (Ulster), Scott Penny (Leinster), Andrew Porter (Leinster), Cian Prendergast (Connacht), James Ryan (Leinster), Dan Sheehan (Leinster), Gavin Thornbury (Connacht), Nick Timoney (Ulster), Kieran Treadwell (Ulster), Josh van der Flier (Leinster).
Source: BBC Rugby Union News