Muldoon was forced off during the second half of the non-cap international and had a scan on his damaged knee yesterday morning.
It is anticipated that the injury will rule him out for 10 to 12 weeks, meaning he will definitely miss Connacht's back-to-back Heineken Cup matches against Biarritz Olympique next month.
Meanwhile, out-half Jonathan Sexton is expected to 'train fully on Thursday' according to Kearney.
Sexton suffered a groin injury in the warm-up before the Fiji game and was withdrawn from the replacements bench, with Keith Earls taking his place.
Kearney said today at the team base in Carton House that a scan on Sexton's groin showed 'no damage'.
"Jonathan Sexton will be fine. He basically got a bit of tightness in his groin. It was more prudent to take him out of the matchday 23 than put him on the bench, although he may have been able to sit on the bench," he explained.
"He had a scan yesterday which excluded damage. He had treatment over the weekend. He feels fine and he'll have modified training today (Monday) and tomorrow.
"Wednesday is a down day for the squad and he is expected to train fully on Thursday. We don't have any major concerns, even minor concerns about Jonathan."
Commenting on the other knocks in the squad, Kearney added: "Tommy Bowe suffered a dead leg in the quad area and didn't train at all last week. It was initially slow to respond but he will train today and tomorrow.
"Eoin Reddan jarred his ankle against South Africa and that has set well. He will train fully today. Donnacha Ryan has recovered from a bug he picked up and will train fully today. Rhys Ruddock is the only other player to be called into the squad."
The Ireland team and replacements for the crunch clash with Argentina will be named on Thursday afternoon.