Ireland head coach Joe Schmidt hopes Johnny Sexton comes through Racing Metro's Top 14 tussle with Toulouse unscathed, after confirming Keith Earls is a serious RBS 6 Nations doubt.
Schmidt fears Earls could be out for as long as two months, thanks to knee ligament damage suffered in Munster's Heineken Cup clash with Edinburgh on Sunday.
"I'd say there might be a bit of medial damage there," said Schmidt, ruling Earls out of Ireland's opening two matches. "If it is, it could be anything from three to six, to eight weeks.
"Certainly he'll be unavailable for the Scotland and Wales games."
Fly-half Sexton will be Ireland's only Six Nations star in club action this weekend.
The bulk of Schmidt's senior players will remain in the national team camp, but France-based Sexton has no such luxury.
Instead he will turn out at Paris' Stade de France on Friday night, with his Ireland boss crossing fingers on the injury front.
France coach Philippe Saint-Andre can withhold 23 of his squad from club action this weekend, in a new initiative.
Ireland are powerless to intervene where Sexton is concerned, but Schmidt admitted he will be watching carefully, with the 2 February Dublin clash against Scotland drawing in.
"I expected it," Schmidt said of Sexton's club commitment. "Racing play Toulouse and he is being taken back for that game.
"He will get back [to Ireland's training camp], hopefully, Saturday evening or Sunday morning depending on his well-being."
Schmidt was forced to admit Ireland are not happy with losing "control" of Sexton during the autumn internationals.
The Ireland boss lamented 28-year-old Sexton playing 13 games in 10 weeks in the autumn approach, eventually succumbing to a hamstring complaint.
Racing rested Sexton for last weekend's 28-3 Heineken Cup hiding at Clermont, but only because they had already bombed out of the competition.
The Paris club's sorry return of one win in the first five rounds of European action meant they could neither qualify for the quarter-finals nor drop into the Amlin Challenge Cup last eight.
All too aware Racing were not doing Ireland any favours, Schmidt still hopes to benefit.
"Jonny's a lot fresher. He didn't play last weekend down in the swamp at Clermont, which I've never seen before," he said.
"So he was a lot fresher. I think he's in really good shape for the Six Nations.
"Certainly, if Racing had still been in the Heineken Cup, he'd have played last weekend and if they'd been playing Toulouse this week he'd have played again.
"The fact that they were out of the Heineken Cup meant that they didn't require Johnny to play and hopefully, touch wood, there's no injury in Toulouse and he'll be ready to go for Scotland."