Glasgow Warriors flanker Chris Fusaro will need surgery on his right ankle after being injured in last Sunday's 19-11 defeat to Toulouse.
Adam Ashe will also be sidelined for several weeks following a neck injury sustained while on Scotland duty.
"Both Chris and Adam have been playing really well and this is a setback for them," said head coach Gregor Townsend.
"We wish them all the best and look forward to welcoming them back into the squad in the new year."
Fusaro underwent an MRI scan to determine the injury, but he will not know how long he will be missing until after the operation.
Glasgow Warriors fell to defeat to Toulouse, who took over the top of European Champions Cup Pool Four.
Duncan Weir's missed conversion attempt prevented the visitors from securing a valuable losing bonus point.