Cardiff Blues will be without Jamie Roberts for their crucial Heineken Cup match at home to Racing Metro 92.
A bounus-point win against the Parisian club on Sunday will take the Blues through to the quarter-finals for the first time in three years.
But centre Roberts will not recover in time from the knee injury that has kept him out of the last three games.
"The knee injury has just plagued him a little bit longer than we thought," said Blues coach Gareth Baber.
"Unfortunately I don't think he's going to be able to win the race to get himself into a position to train this week and partake in the game on the weekend."
Roberts has not featured since limping out of the 28-9 Pro12 win over Newport Gwent Dragons on 23 December and Baber is unsure whether the centre will be fit for Wales' Six Nations opener against Ireland in Dublin on 5 February.
Wales coach Warren Gatland will name his Six Nations training squad on Tuesday.
"There's a little break between our last European game and the Six Nations," added Baber.
"But that will be up to the Union obviously when he joins up with them to see how he progresses."
The Blues put themselves in a strong position to reach the last eight of the Heineken Cup after a 15-22 win at London Irish.
That leaves them level on 17 points at the top of Pool Two with Edinburgh, who won away against Racing Metro on Saturday with the last kick of the game.
If the Blues and Edinburgh finish level on points after the final round of games, the Blues would claim top spot with a superior points total from the two games between the sides.
Both sides are also well placed to reach the next round as one of the two best runner's-up.
"We've given ourselves a chance," said Baber. "But we also know in this competition that can be taken away from you quite cruelly and quickly.
"You've seen that in a number of games this weekend. We've got to make sure we keep that focus all week."
In Roberts' absence against London Irish on Saturday, the Blues selected Gavin Henson at centre for the first time to partner Casey Laulala.
And it was Laulala's individual brilliance that put the Blues on course for victory when he split open the Irish defence to set up Sam Warburton for the only try of the game.
"We try to give the players a structure to allow them to individually express themselves and to create opportunities," said Baber, who, along with Justin Burnell, has been put in charge until the end of the season after impressing in a initial caretaker capacity.
"With people like Gavin [Henson] and Casey [Laulala] on the field that's exactly what you want to do.
"I thought around that we were a little inaccurate in what we did with ball in hand. But that's through no lack of effort - everybody was trying out there to make sure we came away with a victory.
"They will be critical of themselves on Monday - as they always are - and we'll make sure that as a group we progress."
Baber also praised full-back Leigh Halfpenny, who kicked five penalties and a conversion as he continued to take the goal-kicking responsibility from fly-half Dan Parks.
"We've got two world-class kickers on the field," said Baber.
"When you're making decisions on who's going to kick and you speak to the players, Dan is the first to put his hand up to say Leigh has been doing a good job.
"That's the sort of pressure you want. Like any position, you want these boys to pressurise each other and get the best out of each other.
"Leigh was devastated last week after missing one [in the defeat to Leinster], but showed his character and his mettle in this game."