Andy Farrell will be forced into a back-row reshuffle for Ireland's opening Six Nations fixture with Wales on Sunday after Leinster No.8 Caelan Doris returned to his province for assessment of "some symptoms that could be associated with concussion".
The 22-year-old had been tipped to make a big impression on the championship - skipper Johnny Sexton said only last week that a Lions call-up would be no surprise - after he ended 2020 having established himself in the starting pack.
Having finished the Autumn Nations Cup firmly in situ alongside CJ Stander and Peter O'Mahony in Farrell's loose-forward trio, the Mayo native's absence will be keenly felt.
Having been in such fine form for Leinster so far this season, the recalled Rhys Ruddock is one obvious replacement, although a handful of provincial colleagues will be raising their hands too.
Both Will Connors and Josh van der Flier offer options at seven, which would in turn see O'Mahony revert to blindside flanker and Stander to eight.
Another more creative solution could be provided by giving Tadhg Beirne a run in the six jersey with a fit-again Iain Henderson joining James Ryan in the second-row. Beirne was man of the match when Munster narrowly lost to Leinster in Thomond Park last time out, although just two of his last nine international outings have been in the back-row.
Doris wasn't the only casualty yesterday with Quinn Roux returning to Connacht with a neck injury.
"I don't know how long they will be out for," revealed new forwards coach Paul O'Connell.
"Caelan has obviously flagged concussion symptoms, so he has gone back to Leinster. He will go through the procedures and protocols there to get checked.
"Quinn has a neck injury, so he will have to go back and get checked.
"They are two injuries you don't ever want to be messing around with. You have to be careful.
"We have no length of time on either of them. It's frustrating for them and for us.
"Caelan is obviously a fantastic player. I would have worked with him with the Under-20s and he was incredible.
"He has that x-factor and you need those players in world rugby now.
"I played against Quinn towards the end of my time and he was excellent. It's disappointing not having him in now going forward."