Jared Payne appears increasingly unlikely to face England in Saturday's RBS 6 Nations encounter at Twickenham after missing training on Tuesday.
Ireland remain "hopeful" Ulster centre Payne can still beat his hamstring problem in time to start the clash in London, but the 30-year-old must prove his readiness on Thursday.
Head coach Joe Schmidt would typically demand that Ireland's players prove their fitness on the Tuesday ahead of a Saturday Test match.
The Ireland boss broke that self-imposed protocol for the first time ahead of the 10-9 defeat to France, waiting on Simon Zebo and Keith Earls, and could yet follow suit for the England clash, given his appreciation of Payne's defensive qualities.
"Everyone trained fully today apart from Jared Payne," said Ireland's forwards coach Simon Easterby.
"He trained probably 85 to 90 percent yesterday and ran well, but today he's going to do some work with the physios and continue that, and we'll give him until Thursday to prove his fitness."
Ireland initially thought Payne had suffered a dead leg in the France defeat on Feb. 13, but the issue was later revealed as a hamstring injury.
Earls, Stuart McCloskey and Luke Marshall will all fight to slot into the centres in case Payne fails to make the fitness grade.
Munster flyer Earls has recovered from the concussion that kept him out of the game in Paris, and stepped into Ireland's centres when Payne suffered a foot injury during last autumn's Rugby World Cup.
Uncapped Ulster star McCloskey is the form centre, however, with the 23-year-old battering ram causing untold problems for defences with his direct, physical style this term.
Ireland would most likely shift Robbie Henshaw to 13 to accommodate McCloskey in his natural inside centre role.
Schmidt is not keen on pitching Test novices into pressure-cooker scenarios, though, and there are few more ferocious challenges than an international debut at Twickenham.
Ulsterman Marshall has forged a strong provincial partnership with McCloskey this term, moving out to the 13 channel, and he could yet reprise that role with Ireland this weekend.
"I think we've got a few combinations we could run with," said Easterby. "Keith Earls has played there and Luke Marshall has done too.
"I think Robbie Henshaw has shown himself to be fairly adaptable, he's played at 15 this year for Connacht.
"He's well able to be adaptable, and that's what we have to be when we have a few injuries. We need to be adaptable.
"There is a difference defensively between 12 and 13 but we'd feel comfortable that whichever combination we go with that they would have had enough time to gel together."
He added: "We're keen to give Jared the opportunity to come through on Thursday but after that we'll make a decision on what combinations will best fit if Jared doesn't make it."