Leinster back-row Rhys Ruddock has been called into the Ireland squad for Saturday's clash with Argentina.
Muldoon tore his medial knee ligament in Saturday's 53-0 win for the Ireland XV over Fiji in Limerick.
He is likely to be out of action for up to three months, ruling him out of Connacht's back-to-back Heineken Cup clashes with Biarritz next month.
Ruddock, son of former Wales boss Mike, is a straight replacement for Muldoon.
Fly-half Jonathan Sexton should be fit for the Pumas clash despite suffering a groin injury in the warm-up for the Fiji match, and had to skip the game.
Coach Declan Kidney said: "During the warm-up he got a bit of a tight groin so we felt it was prudent to pull him out."
The Ireland team and replacements for the crunch clash with Argentina will be named on Thursday afternoon.