Jamie Roberts faces a race to be fit for the Six Nations after he was ruled out for six weeks with an elbow injury.
The Wales and Cardiff centre suffered a hyperextension during the 14-12 defeat by Australia last weekend and saw a specialist on Wednesday.
He will definitely miss the Blues' Heineken Cup double header against Montpellier and the clashes with Toulon and Sale, and his anticipated return would come very close to Wales' clash with Ireland on 2 February.
Roberts returned to Test match action last month after a long spell out of the game recovering from knee surgery.
He was then concussed in the first-half of Wales' loss to Argentina, before going off nursing a hip injury against New Zealand two weeks later.
His absence adds to a lengthy injury list Cardiff are dealing with ahead of the return to European action, with Wales team-mates Leigh Halfpenny and Bradley Davies also sidelined.
Halfpenny suffered a severe stinger in the match against Australia and Davies was concussed in the same game - they will also miss Sunday's clash with the Top 14 side at the Arms Park.
"Only a fortnight ago things were looking very positive for us, when we were welcoming back some key players," said director of rugby Phil Davies.
"However, we have been extremely unfortunate to pick up a raft of injuries in the last few weeks.
"It is frustrating that we cannot select from a full strength squad for these crucial matches, but on a positive note, it gives a chance for some of our other players to step up and show what they can do."