Japan coach Jamie Joseph has spent the week trying to work out how to get four into three in the back row as he finalizes his lineup for Saturday's Rugby World Cup clash with Ireland, and it seems Michael Leitch could be the one to lose out.

The return from injury of No. 8 Amanaki Lelei Mafi and the form shown by Pieter Labuschagne and Kazuki Himeno in the Brave Blossoms' win over Russia in their Pool A opener means captain Leitch could start Saturday's game at Shizuoka Stadium Ecopa on the bench.

Himeno talked Wednesday as if he would be playing.

"My mental mindset is crucial," he said. "I can't get too tense, too nervous, or too focused. That will allow me to play to my potential, play to my strengths."

And that strength is ball carrying. He showed that in the opening game, when he ran for 121 meters and got across the gain line on six occasions.

"I want to go forward, give momentum to the team so we can play our game. I'll just play with my gut feeling, so if the space presents itself I'll take it. I'm in the middle of the field, so I need to have wide peripheral vision in that respect."

Leitch has been struggling with a groin injury since December and by his own very high standards was slightly off the boil in the 30-10 win over Russia.

This week, during the brief time the media have had access to training, the 30-year-old flanker lined up against what many assume will be the first-choice pack during set-piece drills.

"I don't want to really talk about the players or the makeup of the team. I'd rather you go and watch Ireland and find out more about them," forwards coach Shin Hasegawa said when asked about Leitch.

With Koo Ji-won set for a recall at tighthead prop, the other likely change in the pack will see Luke Thompson start in what will be an emotional game for the 38-year-old lock.

Thompson spent time as a teenager playing for the Blackrock College club in Dublin and made his initial return from self-imposed international retirement against Ireland in 2017.

Elsewhere, Ryohei Yamanaka is set to wear the No. 15 jersey as Japan looks to improve its defense against the high ball.

"The high ball is a very competitive area of the game and potentially the team that handles it the best will win the World Cup," said attack coach Tony Brown.

Kenki Fukuoka trained with the side Wednesday morning but is unlikely to start in order to give his ankle as much time to recover as possible before must-win games against Samoa, which currently tops the pool following its 34-9 win over Russia on Tuesday, and Scotland.

"Kenki is touch-and-go for this weekend but compared to other teams we're in a pretty good position," Brown said regarding the team's injuries, with Scotland in particular hard hit with two players already forced to fly home.

Yu Tamura looks set to play despite an indifferent game against Russia, though the flyhalf was in a subdued mood during the backs' time in the gym.

Much of the talk Wednesday was about the man the Japanese think will be up against Tamura, with Johnny Sexton getting more mentions than all the other players put together.

"I'd say he's a quality player, very experienced in his decision-making and he manages the players around him," said Rikiya Matsuda, who added some much-needed spark to the Japan attack when he replaced Tamura against Russia.

"Sexton controls the players around him. They've got good runners, so he makes use of those runners and he kicks accurately."

The Irish flyhalf is under an injury cloud, however, and Lomano Lava Lemeki said Japan would exploit it if he does take to the field.

"He was injured and has sore legs after the Scotland game, so we'll attack him," said the Japan wing.

And while there are plenty of Irish players still capable of dominating proceedings with the boot, Lemeki is confident Japan can win.

"I think it will be 33-26," he said when asked to predict the score. "Because we train hard and Sexton is not 100 percent."