Leinster back row Dan Leavy will be out for two months after undergoing surgery on a wrist injury.
"It was just one of those things, he landed awkwardly on his hand," assistant coach Robin McBryde told RTÉ Sport ahead of Sunday's Champions Cup clash against Montpellier in the RDS, their first game since 11 December.
"It’s just a freak thing really, unfortunately for him, he’s going to be out for eight weeks so it’s poor luck for him."
Ireland international Leavy, 27, missed out on large swathes of game time following a serious knee injury in 2019 and also required surgery last January, which ended his 2020/21 season.
His most recent cap came in the 2018 November internationals against Argentina and he now misses another Six Nations having last appeared in that year's tournament.
"He was [named] in a team to start week after week and unfortunately he’s picked up an injury," said McBryde. "It’s just the rub of the green."
The flanker underwent a procedure to stabilise a fracture in his wrist last week, said the province.
Captain Johnny Sexton is fit and available, having not played since Ireland’s win over New Zealand in mid-November when he twisted his knee and ankle.
Two unnamed members of the squad are unavailable due to Covid-19, one being a close contact.
"Yeah, everybody has a fit bill of health," said McBryde, when asked about Sexton.
"There are two boys who because of Covid again, who aren't able to train with us but other than that, everybody's got a clean bill of health.
"The numbers at training are brilliant."
Conor O’Brien (hamstring), Dave Kearney (back), Michael Milne (calf) remain out.
Follow Castres v Munster (Friday, 8pm), Connacht v Leicester (Saturday, 3.15pm), Leinster v Montpellier (Sunday, 1pm) and Northampton v Ulster (Sunday, 3.15pm) via our live blogs on rte.ie/sport or on the RTÉ News App, or listen to live radio coverage of Castres v Munster and Connacht v Leicester on RTÉ Radio 1 Extra and RTÉ Radio 1.