Injury doubt Jonathan Sexton must train at least twice this week to be considered to face New Zealand on Sunday, according to Ireland boss Joe Schmidt.
Sexton limped out of Ireland's 32-15 defeat to Australia at half-time on Saturday in Dublin with a hamstring problem.
Head coach Schmidt will receive an update on Sexton's condition on Monday, once Ireland's medics have examined scans.
If the prognosis is unclear, Schmidt admitted Ireland can ill afford to gamble on the 28-year-old Racing Metro playmaker's availability.
Schmidt explained: "I guess on Monday we'll find out the degree of his injury and then that will either make a decision easy for us, because it looks good or it doesn't look good.
"If it's halfway in between, then certainly my expectation would be that he would have to train on Friday.
"He hasn't really spent that much time with us training, he had a little hip-flexor issue plus we left him to rest against Samoa just because of the attritional game time he had already accumulated.
"And I think the wear and tear of that game time has consequences, and unfortunately those consequences have fallen in our lap."
New Zealand are one win away from the perfect season after seeing off England 30-22 at Twickenham on Saturday.
The All Blacks will be desperate to complete a grand slam northern hemisphere tour in Dublin on Sunday weekend and rubberstamp a 100 per cent year to boot.
"He hasn't really spent that much time with us training" -Joe Schmidt on Jonathan Sexton
Steve Hansen's side may have to do without 100-cap fly-half Dan Carter though, who suffered a leg injury of his own in London.
New Zealand's 24-year-old fly-half understudy Aaron Cruden already boasts 27 caps, in contrast to Ireland's out-half back-ups Paddy Jackson and Ian Madigan who have a combined cap total of 11.
Schmidt admitted that experience gap will require careful handling should Sexton miss out this weekend.
Gordon D'Arcy could very well be primed for a midfield recall if former Leinster pivot Sexton is laid low, to offset any potential inexperience at fly-half.
Schmidt added: "One of the things is the number of Tests they've played.
"Aaron Cruden's accumulated almost 30 Tests (whereas) Paddy Jackson and Ian Madigan have a handful each.
"That's a very big gap in terms of learning, experience and opportunity to maybe make some errors and learn from them.
"And they will be surrounded by some experienced players.
"So it's important for us to try to get the right players around the 10 if it's not Johnny."