Hamamatsu - World Rugby player of the year Johnny Sexton sat out training on Tuesday ahead of Ireland's second Pool A match against tournament hosts Japan in Shizuoka on the weekend.

"That wouldn't be unusual for him not to train two days after a Test match," insisted Ireland skills coach Richie Murphy said of the influential but injury-prone playmaker.

"Johnny went through rehab today, but he had also done some work with the team. He's being monitored.

"We expect him to train fully on Thursday."

Sexton, who passed the kicking duties to halfback partner Conor Murray in their opening 27-3 victory over Scotland at Yokohoma on Sunday, picked up a slight thigh injury in that game.

Should Joey Carbery play in Sexton's place on Saturday, he would take on the kicker's mantle.

Kiwi-born centre Bundee Aki and flanker Peter O'Mahony both passed head injury examinations to return to full training, Murphy said, adding that Ireland were taking nothing for granted against what he predicted to be a "very strong" Japan team.

The hosts opened their World Cup campaign on Friday with a comprehensive 30-10 victory over Russia, taking some time to settle their nerves in front of an expectant home crowd.

"We have a lot of respect for Japan," Murphy stressed.

"They are a very, very good side and they've proven that over the last couple of years."

Murphy hinted that head coach Joe Schmidt could ring the changes, but probably not across the board, with further pool games against Russia and Samoa to follow.

"The team we pick is the team we believe will be the right team to play a Test match six days after the last one," Murphy said.

"There could be some rotation, but it will be fully focused on beating Japan."

Munster centre Chris Farrell enjoyed a useful cameo role in the hammering of Scotland and could be in line for a starting spot in the XV to face Japan.

With Aki a shoo-in at inside centre, Robbie Henshaw was seen as Schmidt's first-choice outside him.

But Henshaw has picked up a hamstring injury and is only slowly coming back to full fitness.

Garry Ringrose partnered Aki against Scotland, but fourth-choice centre Farrell impressed when he came on, making headway with ball in hand on several occasions and also proving stout in defence.

"Centre has been quite a competitive area, and we've had a lot of injuries, whether short or long term, but no matter what combination goes in, we connect pretty well," said Farrell.

"We all know each other's roles and we feed off each other, so we're very comfortable no matter who's there."

Farrell said the game against Japan would be an "unbelievable event".

"It's massively exciting for us this weekend because we're going to play the host nation in a massive tournament like the Rugby World Cup in front of a massive home crowd.

"We have to make it work for us."