Saracens will be without England star Alex Goode when they go to Nantes for their key Pool 1 clash with Racing Metro 92 at Stade de La Beaujoire on Saturday.
The international full back, an ever present in Saracens' Heineken Cup campaign this season that has seen them take a two-point advantage at the top of the group ahead of the French club, has scored 35 points in his 22 European appearances.
But a shoulder injury means he will miss both the Nantes contest and the home tie against Edinburgh Rugby eight days later.
"Alex has a tear in his shoulder," said Saracens Director of Rugby Mark McCall.
"At the minute we are looking at a non-surgical route and there's a chance he could be fit in two or three weeks' time but it all depends how his rehab goes over the next couple of weeks."
And Saracens have been warned they face a team in prime form - Racing ending Toulon's 18-match winning home run with a 19-15 Top 14 victory on Sunday night.
Goode has not played since December 16 and the home win over Munster and also faces a race against time to be fit for England's opener against Scotland at Twickenham on February 2.
Meanwhile Saracens go into their final decisive European matches with England outside half Owen Farrell having extended his perfect goal-kicking streak to 20.
"Obviously Owen's goal kicking was good," said McCall. "But defensively he was magnificent because Sale came down his channel with some pretty heavy weapons."
