Pivotal centre Jared Payne and outstanding flanker Sean O'Brien sat out training on Friday deepening Ireland's injury worries on the eve of their Test with Rugby World Cup 2015 finalists Australia.
Both have "lower limb tweaks", according to defence coach Andy Farrell, but Ireland can ill afford to lose them as they are already without playmaker Jonathan Sexton and Payne's centre partner Robbie Henshaw.
Simon Zebo, CJ Stander and Rob Kearney also missed parts of training this week after taking heavy blows in the bruising clash with New Zealand last Saturday in Dublin.
New Zealand-born Payne would be a huge loss as the other first-choice centre Henshaw - the pairing that were so effective in the historic 40-29 win over the world champion All Blacks in Chicago three weeks ago -had already been ruled out with concussion.
Henshaw has been replaced by the talented but raw 21-year-old Garry Ringrose in the starting line-up.
Uncapped 22-year-old Rory Scannell has been lined up to be promoted into the matchday squad should Payne fail to recover, although it is likely Keith Earls would move into the centre with Zebo filling in on the wing.
O'Brien has had an injury-plagued career - he only returned to the national side last Saturday for the 21-9 defeat by the All Blacks at Lansdowne Road - and the experienced Peter O'Mahony has been kept on in case he isn't fit.
"We've had a couple of guys that have had a couple of niggles, and the captain's run is normally just a stretching legs type session," said Farrell, referring to eve-of-match training session.
"We made a couple of decisions this morning to rest Jared Payne and Sean O'Brien and see how they are in the morning. - Andy Farrell
"We'll have a discussion in the morning about their situation.
"They both have just lower limb tweaks, that's what normally happens during the week.
"You pick things up on the way from the game, and etcetera.
"The main session was yesterday and we decided to give a couple of players more time.
"We kept Peter O'Mahony here, and we've got Rory Scannell as well covering for Jared Payne.
"All the main work's been done already, that's why we are resting them, to give them every chance to feel 100 per cent."
Former England assistant coach Farrell denied the raft of injuries had disrupted the Irish rhythm this week and left them exposed to an Australian side that has won all three of its Tests so far on their northern hemisphere tour.
"No it isn't, because this is what we've been building to for the last year," said Farrell.
"We've been building strength in numbers, strength in depth is getting bigger and better all the time.
"We've seen that across the last year and on the South Africa tour and we're seeing it again."
Australia takes on Ireland at 4:30am AEDT Sunday, LIVE on SBS and beIN Sport (Channel 515).