Johnny Sexton didn't train fully today in Ireland's first session since their opening World Cup win over Scotland.
The Ireland management have said that Sexton, 34, will do so on Thursday ahead of Saturday's second pool game against Japan.
Sexton had suffered a bang on the quad in the first half of the 27-3 win over the Scots and Conor Murray took over the kicking duties off the tee, converting one from three kicks at the posts.
Joey Carbery looks favourite to wear the no 10 shirt in Shizuoka after training fully since late last week, having recovered from minor ankle surgery.
"Johnny just went through sort of rehab today, did some work with the team, but is being monitored," said Ireland kicking coach Richie Murphy.
"He'll train fully on Thursday and that wouldn't be unusual for him.
"He took a bit of a bang, a knock on the quad, and he just felt uncomfortable. We had no problem with Conor taking on the kicking duties, obviously he's disappointed he missed two but that's the way it goes.
"Joey is fine, Earlsy (Keith Earls) and Rob Kearney fully trained today - even by the end of last week they were ready to play but we didn't feel it was the right thing to put them in."
Positive news for Joe Schmidt is that Bundee Aki and Peter O'Mahony have both passed HIA 3, which meant they were able to resume full training today.
Robbie Henshaw did a lot of running during the session and will continue to be monitored, but could resume full training this week.
Japan are bullish about their chances of causing a big upset and Murphy said that it was important that Ireland imposed themselves on the game early.
"We have a lot of respect for Japan, they're a very good side," said Murphy. "There could be some rotation in terms of our team, but Japan in Japan is a very difficult game.
"They will try and play fast and loose, but with structure inside that. It will be up to use to implement our game plan over them.
"If the game breaks up, it breaks up, you've got to be able to survive - but we want to stamp our game on them".
