Ireland fly-half Johnny Sexton has been ruled out of the team to face Argentina in Sunday's World Cup quarter-final because of injury.
Sexton, who went off with a groin injury during last week's 24-9 victory over France, had trained during the week and was initially named in the Ireland team but suffered a setback in training yesterday.
Ian Madigan comes in to replace Sexton, while Paddy Jackson joins the replacement's bench.
A statement from the IRFU confirmed: "Post training yesterday Johnny Sexton tightened up and although scans earlier in the week were normal a further scan yesterday afternoon revealed a very minor adductor strain.
"It was therefore felt that it was better to withdraw him from the game rather than risk further injury. Ian Madigan will start at out-half and Paddy Jackson comes onto the bench."
Sexton hobbled out of Ireland's victory over France in Cardiff last weekend after just 25 minutes.
The 30-year-old Leinster lynchpin immediately turned his attention to trying to beat that groin problem in time to face the Pumas.
Ireland boss Joe Schmidt insisted Sexton was 100% fit on Thursday when naming the former Racing Metro fly-half in his starting line-up.
But that groin problem clearly lingered, and Ireland have moved quickly on Saturday to rejig their backline and hand Madigan time to come to terms with his new weekend challenge.
Ireland are still yet to reach a World Cup sem-final, and contest their sixth last-eight encounter this weekend (1pm, live on RTÉ Radio 1).
Madigan came of Test-match age against France, bossing Ireland's backline with poise and verve.
The 26-year-old playmaker must now repeat that feat, but this time from the off, against the fluent and threatening Argentina side that have lit up the World Cup with their ebullient counter-attacking style.
Meanwhile, Isaac Boss has been brought in to the squad as further scrum-half cover in place of Jared Payne. Madigan had also been operating as back-up scrum-half behind Conor Murray and Eoin Reddan.
