Leinster's Rhys Ruddock has been drafted into the Ireland squad ahead of the final Guinness Series match against Argentina on Saturday.
It was confirmed today that John Muldoon tore his medial knee ligament during last 53-0 win for the Ireland XV over Fiji in Thomand Park.
Muldoon will be ruled out for up to three months, including Connacht's two Heineken Cup clashes with Biarritz in December.
Ireland team manager Michael Kearney has said Jonathan Sexton is expected to return to full training on Thursday.
Sexton suffered a groin injury in the warm-up before the Fiji game and was withdrawn from the replacements bench, with Keith Earls taking his place.
The Irish team for the vital match with Argentina in the Aviva Stadium will be named on Thursday.