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) during Sunday's Scotland victory, lasting just 25 minutes of the Yokohama clash.
The British and Irish Lions star has since passed all head injury checks and did not suffer a concussion, however, and he returned to full training on Tuesday.
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."
Listen to the RTÉ Rugby podcast on Apple Podcasts, Soundcloud, Spotify or wherever you get your podcasts.
Follow Ireland v Japan on Saturday (kick-off 8.15am) via the live blog on RTÉ.ie/Sport andthe NewsNowApp, watch live on RTÉ2 from 7.30am or listen to live match commentary on RTÉ Radio 1.
