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Johnny Sexton has been ruled out of Ireland's World Cup quarter-final clash with Argentina, after aggravating his groin problem in training on Friday.
Ian Madigan will start Sunday's last-eight battle with the Pumas in Cardiff, with Paddy Jackson taking a seat on the bench.
Scrum-half Isaac Boss has been drafted into Ireland's squad to replace the injured Jared Payne, to offer extra half-back cover.
"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," read an Ireland statement.
"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.
"Isaac Boss has been brought into the squad as further scrum-half cover in place of Jared Payne.
"With Ian Madigan now starting tomorrow's match versus Argentina it's ideal for him to focus fully on the out-half position.
"The centre position vacated by Jared (Payne) is well covered by Robbie Henshaw, Keith Earls, Darren Cave and Luke Fitzgerald."
Sexton hobbled out of Ireland's 24-9 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 per cent 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.
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.