Jack Conan could be facing four months on the sidelines after undergoing foot surgery.
The Leinster back-row forward was forced to return home from the Rugby World Cup after suffering a foot injury in the lead-up to Ireland's second Pool A game with Japan.
Conan had come off the bench in the win over Scotland and was due to start the game with the Brave Blossoms, before injury struck and eventually saw his place in the squad go to Jordi Murphy.
Leinster confirmed that the number-8 was undergoing a procedure this week, and they hope to offer a further prognosis later this week.
After seeing a specialist, the 27-year old needs to go under the knife in London and could be out for up to four months.
Such a spell on the sideline would render Conan doubtful for the start of Ireland's 2020 Six Nations campaign at home to Scotland on February 1st.
Conan will also miss at least four rounds of Leinster's Heineken Champions Cup pool campaign which begins on November 16 against Benetton at the RDS.
Leinster prop Ed Byrne is also facing a spell on the sidelines, having missed the province's first two games of the Guinness Pro 14 season.
Byrne is suffering from a neck injury, which head coach Leo Cullen says is taking its time to heal, "It has just been a bit slower than we thought.
"He got a bang on his neck and it just hasn’t quite settled down.
“We were hoping he was going to be involved over the last couple of weeks but it has been pushed out again.
"The break next week and hopefully he’ll be good to go after that. But, again, we can’t say for certain."
Cullen is already without the services of Barry Daly (knee), Ciaran Frawley (ankle) and Dan Leavy ahead of Friday's Pro 14 clash with Edinburgh.
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