Ireland fear Jack Conan's foot injury could prove a World Cup "setback" for the Leinster back-row forward.
Conan was meant to start Ireland's Pool A clash with Japan in Shizuoka on Saturday but suffered a blow to his foot in training on Thursday.
Peter O'Mahony has stepped into the starting line-up leaving Ireland selecting the same pack that subdued Scotland 27-3 in Yokohama last weekend.
Ireland hope Conan will be fine to work back to fitness quickly, but scrum coach Greg Feek has admitted the 27-year-old will not know the full extent of his injury until after Saturday's match.
"At this stage, Jack is just a wait and see," said Feek.
"We're going to get some opinions on that over the next couple of days, and go from there.
"It's a little bit of a setback at this stage, but once we get the game out of the way and he gets fully assessed we'll have some more information on that. So probably after the game I'd say."
Munster flanker O'Mahony failed a Head Injury Assessment (HIA) in Saturday's Scotland victory, lasting just 25 minutes of the Yokohama clash.
The British and Irish Lions star passed all head injury checks and did not suffer a concussion however, and returned to full training on Tuesday.
And now O'Mahony has admitted he is itching to get back into action having proved his readiness to return to the Ireland line-up.
"It's not something you plan, obviously it's my second World Cup but the first game of the tournament and it wasn't ideal to be coming off that early," said O'Mahony.
"But, it's part and parcel of rugby these days. You get through your return to play stuff, the S&C guys and the medical team are well used to it at this stage, it's bread and butter for them and they were very good.
"I passed all the protocols, I trained fully on Tuesday and I'm ready to go.
"I'm feeling fresh, I only got 25 minutes or whatever so I am looking forward to playing."
Japan prop Yusuke Kizu and then their head coach Jamie Joseph have this week hit out at Ireland's scrum, insisting Joe Schmidt's side can be prone to bending the rules at the set-piece.
Scrum coach Feek shrugged off that suggestion however, claiming head coach Schmidt has turned Ireland into the most disciplined Test team of the last 20 years.
Asked about Kizu's scrum comments, Feek said: "That was one of the younger players in the squad and it can be daunting coming out to do media.
"And we've all been in that position where sometimes you say things.
"But for us we pride ourselves as a team, and it's something that Joe has driven as one of our mantras right from the start with Ireland is to be the most disciplined team in world rugby.
"And to be fair to the players, after working with them for so long, how they go about their business in terms of their discipline is world class.
"Some of our discipline, I would say in the last 20 years you wouldn't see a team that's so hard on their discipline.
"There's always going to be the odd occasion where two front-rows might not agree on things, or the referee might not get it right, but that's all part of the game.
"And the media as well, you might read stuff yes, but in this type of competition you just have to be focused on our systems, our processes and backing our players to do the thing that they are most used to doing, and that's being disciplined rugby players."
