Hopes dashed: Last-minute loss of Johnny Sexton a massive blow for Ireland ahead of Argentina clash
Ian Madigan to replace injured 30-year-old fly-half in do-or-die match against Pumas
Ireland fly-half and key man Johnny Sexton was ruled out of Saturday's Rugby World Cup quarter-final against Argentina after a groin injury flared up again.
The 30-year-old - who went off with the injury in the first half of Ireland's 24-9 victory over France last Sunday - will be replaced in the starting 15 by Ian Madigan, who performed well when he came on as a replacement against Les Bleus.
"It was therefore felt that it was better to withdraw him from the game rather than risk further injury."
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The 35-year-old New Zealand-born Boss - capped 23 times since making his test debut in 2006 - was originally left out of the squad because Ireland coach Joe Schmidt considered that Madigan was able to provide cover at scrum-half.
Iain Henderson, having an outstanding tournament, comes into the second row to partner Devin Toner, with Jordi Murphy and Chris Henry filling the flanking vacancies left by O'Mahony and O'Brien. Jamie Heaslip captains the side from number eight.
Argentina reached the last four in the 2007 World Cup, having beaten Ireland in the pool stage and they have looked a dangerous team this time around.
Having gone down 26-16 to pool C winners New Zealand in their opener, they pulverised the rest of the group opposition.
Pumas scrum-half Martin Landajo is one of a host of regulars recalled by coach Daniel Hourcade after a second-string side posted a 64-19 win over Namibia in their final pool game last week.
Landajo will be partnered at half-back by Nicolas Sanchez, and veteran Juan Martin Hernandez in midfield alongside Matias Moroni, playing in the absence of the suspended Marcelo Bosch.
"It's a do-or-die game, very tough and tricky," said Hourcade. "It's the game we were all hoping for, for which we were preparing."
Hourcade said Ireland missing a host of first-choice players would "change nothing".
Ireland had a slight hiccup against Italy in pool play, a hard-fought match eventually going their way 16-9.
Schmidt said he and his coaching team had taken good note.
"[Argentina and Italy] tend to get in your face, challenge you at the breakdown and on top of that Argentina have the luxury of some world-class finishers and some people very adept at transferring the ball to those finishers."