Johnny Sexton did not train fully on Tuesday, but Ireland still expect him to be fit to face Japan on Saturday in Shizuoka.
Skills coach Richie Murphy insisted it "wouldn't be unusual" for fly-half Sexton to sit out a training session just 48 hours after playing in a Test.
Ireland hope British and Irish Lions playmaker Sexton will return to full training on Thursday, having sustained a minor quadriceps niggle in Sunday's 27-3 win over Scotland in Yokohama.
Murphy also revealed Peter O'Mahony and Bundee Aki have passed their head injury checks and returned to full training, leaving both available for the Japan clash.
Murphy said: "Johnny went through rehab today, but he had also done some work with the team; but he's being monitored.
"We expect him to train fully on Thursday. That wouldn't be unusual for him not to train two days after a Test match."
Robbie Henshaw continues to step up the recovery from his hamstring problem that kept him out of the World Cup opener.
Murphy revealed Leinster's powerhouse centre is back running, and will be reviewed again in the next 48 hours.
While Henshaw is unlikely to be ready to face Japan this weekend, Ireland continue to make positive noises about his recovery.
Murphy said: "Robbie was in training today; he's in his return to play phase now. He did a lot of running today and came through that no problem.
"We'll see what he's like on Thursday and make a decision on him them."
Keith Earls, Rob Kearney and Joey Carbery are all fit and available for selection for Japan, leaving Henshaw Ireland's only major injury concern - should Sexton recover from his thigh knock.
Murphy added: "Earlsy, Rob Kearney and Joey have all made really good progress, and it's a good situation.
"They were all at the point at the end of last week where they were effectively ready, but we didn't want to take any risks."
Ireland can move a long way towards a quarter-final with victory over Japan on Saturday, but Murphy insisted Joe Schmidt's men were taking nothing for granted despite the hosts being ranked just ninth in the world.
Ireland are likely to mix up selection to face the Brave Blossoms, but Murphy insisted changes are not expected to be wholesale.
Murphy said: "The first thing is, we've a lot of respect for Japan. They are a very, very good side and they've proven that over the last couple of years.
"The team we pick will be the team we believe will be the right team to play a Test match six days after the last one.
"There could be some rotation in it but it will be fully focused on beating Japan."