The 30-year-old picked up the injury during Ireland's victory over France in their final group match last weekend and initial thoughts were that he would not be fit enough to face the Pumas.
But, as the week progressed, it began to emerge that the injury was not as bad as first feared and he might be able to take his place in the side.
But, after being named in Joe Schmidt's team on Friday, he was then withdrawn on Saturday morning with Ian Madigan taking his place.
Ulster fly-half Paddy Jackson has been added to the match-day squad to take Madigan's place on the bench.
The Irish released a statement on Saturday outlining the reason why Sexton has not been risked.
"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 read.
"It was therefore felt that it was better to withdraw him from the game rather than risk further injury."
Iain Henderson comes into the second row to replace the injured Paul O'Connell, with Jordi Murphy and Chris Henry selected in the back row.
Ireland: Rob Kearney; Tommy Bowe, Keith Earls, Robbie Henshaw, Dave Kearney; Johnny Sexton, Conor Murray; Cian Healy, Rory Best, Mike Ross; Devin Toner, Iain Henderson; Jordi Murphy, Chris Henry, Jamie Heaslip.
Replacements: Richardt Strauss, Jordi McGrath, Nathan White, Donnacha Ryan, Rhys Ruddock, Eoin Reddan, Ian Madigan, Luke Fitzgerald.