Rory Best will miss the final two pool matches against France and Argentina having suffered and dislocated thumb in Ireland's narrow win over Georgia.
The Ulster hooker also cracked the bone, ruling him out for up to three weeks. Unless Eddie O'Sullivan's side qualify for the quarter-finals, which is looking increasingly unlikely given the performances to date, the injury will spell the end of Best's campaign.
Best's absence should see Jerry Flannery come into the starting line-up for Friday's all-or-nothing showdown at the Stade de France while Frankie Sheahan is likely to be called into the replacements.
Elsewhere, Denis Hickie will miss training tomorrow as he recovers from a bruised knee while John Hayes also picked up a slight calf problem.
O'Sullivan, meanwhile, was at a loss to explain how Ireland's performance levels have dipped so remarkably in the tournament.
"We haven't done anything different in training. The work the lads are putting in is right up there. As ever, we've been assiduous in our preparation," O'Sullivan said.
"I wish I could tell you why we're not firing on all cylinders. We just haven't clicked, but I don't have a magic answer as to why that is," he told reporters.
"I wouldn't say it's mission impossible," he added of the match that France must win after their Pumas debacle if they are to reach the last eight of the tournament they are hosting.
"The lads felt better about things than they did after last weekend's game against Namibia," he said of Ireland's unconvincing 32-17 win over Namibia.
"However, our form has been fairly stagnant and if we keep turning the ball over we're going to be in trouble whoever we play. If you keep turning the ball over, you're not back to square one, you're back to square minus one."
O'Sullivan took heart from the fact that France are a known quantity for Ireland.
Ireland had beaten Georgia easily in two previous meetings but the Georgians have improved even from their first to second match at this World Cup, having gone down 33-3 to Argentina on Tuesday in Lyon.
"France are a very good team, but in a way at least we know what we're up against this time," O'Sullivan said. "Georgia were very unpredictable and awkward to play against, but that's something we've got to learn to deal with.
"We're trying to pursue a style of rugby that's served us well. It's imperative to do that if we are to succeed at the highest level," O'Sullivan said.
"We can't just revert back to whacking the ball into the air and hoping to force errors out of the opposition. That won't get us big results," he added.
"It's a high-risk strategy and you pay a high price if you get it wrong, but we're trying to play a style that we've developed over the last year and a half."
