Leinster loose forward Rhys Ruddock has been drafted into the Ireland squad ahead of Saturday’s Test against Argentina.
Ireland team manager Michael Kearney confirmed on Monday that John Muldoon tore his medial knee ligament during last Saturday's 53-0 win over Fiji in Limerick.
Muldoon was forced off during the second half of the non-cap international and had a scan on his damaged knee on Sunday morning.
It is anticipated that the injury will rule him out for 10 to 12 weeks, meaning he will miss Connacht's back-to-back European Cup clashes with Biarritz Olympique next month.
Meanwhile, flyhalf 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Sexton suffered a groin injury in the warm-up before the Fiji game and was withdrawn from the replacements bench, with Keith Earls taking his place.
Head coach Declan Kidney explained afterwards: "During the warm-up he got a bit of a tight groin so we felt it was prudent to pull him out."
Kearney said at the team base in Carton House on Monday that a scan on Sexton's groin showed 'no damage'.
The Ireland team for the crunch clash with the Pumas will be named on Thursday afternoon.
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