Staunton replaces the injured Paddy Wallace, who found it difficult to kick the ball during last weekend's 22-20 defeat to the Pumas and was subsequently substituted during the second half.
O'Sullivan is expected to make more than 10 changes to a starting XV he will announce on Tuesday.
Ulster duo Andrew Trimble (ankle) and Tommy Bowe (shoulder) both picked up knocks during the first Test defeat and there are question marks hanging over their participation.
Leo Cullen was ineligible in Santa Fe owing to his participation in Leicester Tigers' Heineken Cup final defeat to London Wasps, but the lock is expected to be included in Ireland's starting line-up in Buenos Aires.
And the 29-year-old insists the clash is an important test for what is expected to be a new-look Ireland side.
"Argentina is always a big challenge as they have wonderful players. We can't relax as Saturday's match proved. The people in the stands won't allow that," he said.
"We have to show character and attitude in each play. The most important thing is to respect our way of playing, leaving our heart on the field and enjoying the match.
"This game is going to be the last of the season ahead of the World Cup, so we'll have to be careful and try to beat Argentina in every position on the field."
Cullen was left with a sour taste in his mouth after Felipe Contepomi scored the winning drop-goal in injury time at the weekend.
But he believes that his team-mates are capable of exacting revenge in Buenos Aires.
"The team is in a pretty good form, so we know we'll do our best on Saturday," he added.