Jared Payne's foot injury could end up laying him low for Ireland's crucial World Cup clashes against Italy and France.
The Ulster centre is still suffering bruising and inflammation to his foot and has failed to recover in time to face Italy at London's Olympic Stadium on Sunday.
Full-back Rob Kearney is expected to be fit to face France on Sunday week - but has not beaten his gluteal strain quickly enough for the Italy clash.
Simon Zebo will start against Italy with Keith Earls at outside centre, where victory will secure Joe Schmidt's side qualification for the quarter-finals.
"Jared's progressing really well; his foot was scanned as a precaution to make sure there was nothing structurally awry," said head coach Schmidt.
"He's got bruising, with inflammation and discomfort but that's getting better by the day. He can walk and jog freely now.
"It's just the frustration that he hasn't been able to train fully.
"We'd be quietly confident at the moment that he will be ready for France.
"We'd expect he'd be able to train with the team on Wednesday.
"Rob's probably fit to play. But he wasn't fit to play at the start of the week.
"He trained pretty much fully today, so there are no real doubts on him for next week."
Ireland will take on France in Cardiff on Sunday, October 11, one week after their Italy encounter.
The winners of that Millennium Stadium battle are most likely to top Pool D and face an anticipated quarter-final with Argentina.
Payne's readiness to face France could now hinge on Ireland's training session on Wednesday next week.
Schmidt expects the Ulster star to be ready for that session and will need him to take a full part to then warrant selection for Les Bleus.
"One of the difficulties is trying to make an assessment when you're waiting for a soft tissue injury to heal and trying to get a bit of clarity for selection," said Schmidt of the trials of gauging players' fitness.
"I think Jared's importance would be reflected by his combination with Robbie Henshaw when he has played.
"They made a really good combination during the Six Nations.
"Jared's really disappointed not to be playing this weekend, because he misses an opportunity and offers another opportunity to someone in good form.
"We're just trying to get him right as soon as we possibly can."