Steve Borthwick's 'significant pectoral injury' could deny the Saracens captain the chance to close a decorated career with back-to-back Heineken Cup and Aviva Premiership finals.
Former England captain Borthwick must train on Thursday to prove his fitness for Saracens' double final fortnight.
Saracens rugby director Mark McCall rated the 34-year-old lock as '50-50' to recover in time for Saturday's European showpiece battle with Toulon in Cardiff.
Former Ulster boss McCall admitted if Borthwick does not make the Heineken final then he will also miss the Premiership final on May 31.
"He's made some progress actually this week so I suppose we're optimistic, but we'll make a decision towards the end of the week," said McCall on his captain, who trudged out of Saracens' 31-17 Premiership semi-final victory over Harlequins.
"He's got a chance. He hasn't trained yet and there's a few things he's got to do before he's declared fit, but we'll make a firm decision on Thursday.
"We wouldn't play him unless he's proven he can play, functionally he's got to be strong enough to play.
"We wouldn't gamble at all with him. He's got a significant injury to his pectoral muscle."
Skipper Borthwick will end his 16-year top-flight career this summer and McCall conceded he could already have made his final professional appearance.
Springbok Alistair Hargreaves and England's Mouritz Botha would start against Toulon should Borthwick miss out.
Botha pulled out of last weekend's Harlequins victory at the last minute with a back problem, but McCall said he is fit to face reigning champions Toulon.
"It would just prove that life's unfair, because no one deserves to play in a Heineken Cup final and a Premiership final in his last two games of rugby ever more than Steve Borthwick to be honest," said McCall.
"So fingers crossed that will happen.
"Already there's no self-pity with Steve, he's been massively involved in the preparation in the last two days, so he'll have a massive contribution to make whatever happens.
"He's been so wonderful for us as a leader, he's been brilliant for us of course.
"We want him to play in a game like that for us, a guy with big-match experience who is a tremendous leader in every respect. But if he doesn't play we've just got to get on with it.
"We think we've got a lot of other people who provide a lot of leadership and direction.
"The best part of Mo's back seizing up on Saturday was that Alistair Hargreaves got 80 minutes, which he actually needed now that he looks like he'll be very involved on Saturday.
"He played really well, he got stronger as the match went on, and he's a real class player Alistair."
McCall refused to be drawn on the RFU sending Chris Ashton a letter after he was heard to shout 'push it' as Nick Evans attempted a conversion for Harlequins in Saturday's semi-final.
"It was a short investigation," said McCall.
"It's been dealt with and we move on, that's all I'm going to say about it.
"There are a lot of things I could say, but I'm not going to. It's been dealt with and we move on."