Chris Farrell will miss Ireland's clash with Russia on Thursday after being ruled out with the concussion that he suffered during yesterday's shock upset to Japan.
The centre was replaced just after the hour mark and after failing his HIA, didn't return to the pitch.
Rob Kearney and Tadhg Furlong also suffered head knocks but both have passed the necessary protocols and as it stands, along with the fit-again Johnny Sexton, are expected to be available for Ireland's third pool game.
Joe Schmidt had planned to make a host of changes, but he now faces a selection headache in terms of his midfield.
Garry Ringrose has already played both games against Scotland and Russia, and could have been pencilled in for a breather.
Meanwhile, despite upping his training load, Robbie Henshaw remains a doubt as he hasn't yet fully overcome the hamstring injury that has seen him miss both games thus far.
"Robbie ran really well yesterday but we’ll obviously have to take a view on his selection when we get back to training," forwards coach Simon Easterby said after the squad arrived at their new base in Kobe.
"But he’s doing really well – and (we are) really pleased with his progress."
Bundee Aki is set to come back into the team after missing the Japan game with a head injury. Jordan Larmour and Keith Earls could fill in at outside centre if called upon.
"Chris Farrell had a confirmed concussion so he won't recover in time for Thursday," Easterby continued.
"Rob Kearney passed his HIA2, so he's within that process. Tadhg (Furlong) came back onto the pitch, so he's fine as well. Those were the three obvious ones from a head injury perspective."
Sexton is also in line to return to action having not been risked against Japan. The out-half picked up a quad issue in the win over Scotland, which hindered him kicking shots at goal.
Sexton insists that he is raring to go this week as he looks to bounce back from the setback.
"One thing about this team is that we back the squad, the 31 guys," he maintained.
"When we picked the squad, we said that everyone that was here was ready to play. I picked up that strain against Scotland and I wasn't going to train Monday, Tuesday. The preparation time is so short in a six-day turnaround that the guys made a decision for me to sit this one out.
"You want to play every game, especially in a World Cup, but hopefully I can pay back the whole squad with a good performance this week and beyond."
Sexton cut a frustrated figure as he was asked for his take on why Ireland had got it so horribly wrong against the host nation.
"We've got to look at ourselves and say the discipline wasn't good enough," the out-half added.
"There's a lot of access points we gave them, discipline would be top of the list. We had them under pressure a couple of times and we let them out.
"We didn't exit a couple of times as well as we should, so there's one or two things in each department that added up, and you have to give a very good team like Japan a lot of credit on the day as well.
"There's lots of little things that we don't normally do that we did yesterday.
"We've got to figure out what it happened and I suppose the only blessing in disguise is that, in the last two World Cups that I've been involved in, we've not cruised through the group but we've had everything go our way in the pool stages and then we've had the day that we had yesterday in a quarter-final and we'd be going home today.
"The great thing now is that we've got the rest of the pool to get things together. We know, to a certain extent, that we've got things in our own hands in terms of if we can win the last two games and score a number of tries we can definitely qualify for the quarter-final.
"If that's in first or second place, that's out of our control, so we won't worry about that, that's done now, so it's up to us now to put in two big performances and hopefully put ourselves into a quarter-final and, we know if we do that, then we've got a chance."