Chris Whitaker could be absent for the defining part of Leinster's season after the Australian scrumhalf sustained a leg fracture in Sunday's disappointing 33-6 defeat to Toulouse at the Stade Ernest Wallon.
A full medical report will be released today but the worst-case scenario has the 33- year-old sidelined for 10-12 weeks.
The initial out-until-mid-February prognosis means Leinster's interests in Europe could be concluded by his return as the pool stages finish up with a trip to Leicester on the weekend of January 18-20th.
Whitaker was legitimately tackled by French flanker Thierry Dusatoir around the lower right leg on 20 minutes. He had a plate inserted yesterday, which will be removed in six weeks, at which point he can begin rehab.
"Chris has a broken leg," said coach Michael Cheika on arrival back in Dublin airport at lunch-time yesterday.
"We'll wait for a proper medical assessment and release it tomorrow. I don't want to make an assessment and then get it wrong. The same applies to Gordon (D'Arcy, hip) and Girvan (Dempsey, knee)."
D'Arcy and Dempsey were both unable to continue around the hour mark, by which stage the contest was already decided as Toulouse led 21-6 and were aggressively pursuing a bonus point.
Subsequent touchdowns from wingers Cedric Heymans and Vincent Clerc, four minutes into injury-time after a missed tackle by replacement Christian Warner, secured what may prove a vital extra point.
D'Arcy is expected to be available for the home and away matches against Edinburgh, starting at the RDS on December 7th, but Dempsey's recurrence of damaged medial ligaments could conceivably sideline him until the New Year.