Japan on Thursday left captain Michael Leitch out of its starting team for Saturday's clash with Pool A favorite Ireland as a fit-again Amanaki Mafi took his place in the back row as one of five changes from the Brave Blossoms' opening-day win over Russia.
With winger Kenki Fukuoka still unavailable with a calf injury, Ryohei Yamanaka comes in at fullback as Will Tupou shifts to the wing and Lomano Lemeki drops to the bench.
Elsewhere in the pack, Luke Thompson replaces Wimpie van der Walt in the second row, Koo Ji-won and Shota Horie come into the front row and Kazuki Himeno moves to flanker to make room for Mafi at number eight.
South Africa-born flanker Pieter "Lappies" Labuschagne will captain the side in Leitch's absence.
"What I believe around the game, in terms of our bench, we've got to have an impact," Japan coach Jamie Joseph said of his decision. "If we're going to be in a position to win the match it's going to come down to the last five or 10 minutes, and we'll need clear leadership."
Joseph said Leitch's injury-plagued year had not been a factor, stressing how well Himeno and Labuschagne had played in the opening night 30-10 win over Russia.
"We have a lot of quality loose-forwards and in-form players. So we have in-form players playing really well, and experienced players coming on. So as a coach I get to have both things," he said.
Earlier, Ireland coach Joe Schmidt named a side showing four changes to the one that started the 27-3 defeat of Scotland last Sunday.
Jack Carty replaces the injured Johnny Sexton at flyhalf, and there are recalls for the experienced Keith Earls on the wing and Rob Kearney at fullback while Chris Farrell comes in at inside center.
Schmidt had also hoped to play Jack Conan but the flanker was injured in training so Peter O'Mahony will once again wear the No. 6 jersey despite taking a knock to the head last week.
"We always knew we were going to have to manage this start of the tournament because of a six and a five-day turnaround," Schmidt said of his selections.
"So whether guys play this week or next week, we do know that the challenge this weekend will be particularly tough but we also know that we've got to show confidence in the squad, we have that confidence."
Losing Sexton was not, Joseph said, a big blow to the men in green given the depth of their squad. And besides, he added, Japan was focusing more on what it needs to do rather than what the Irish will bring to the game.
"I don't think we're the sort of team that can afford to be worried about whether Johnny Sexton is playing or not," he said.
"You don't get to be No. 1 in the world with one or two quality players. While that is a surprise it doesn't change anything that we need to do."
Thompson was the team leader in tackles the last time the teams met back in 2017 and the veteran said "defense would be key" on Saturday as Japan looks to keep alive its hopes of top-eight finish.
"They are very organized and good at keeping the ball," he said. "When we have it we need to be patient and keep it. We also need to stick to our systems and be disciplined."
Japan lineup
15-Ryohei Yamanaka, 14-Kotaro Matsushima, 13-Timothy Lafaele, 12-Ryoto Nakamura, 11-Will Tupou, 10-Yu Tamura, 9-Yutaka Nagare, 8-Amanaki Lelei Mafi, 7-Pieter Labuschagne, 6-Kazuki Himeno, 5-James Moore, 4-Luke Thompson, 3-Koo Ji-won, 2-Shota Horie, 1-Keita Inagaki
Replacements: 16-Atsushi Sakate, 17-Isileli Nakajima, 18-Asaeli Ai Valu, 19-Wimpie van der Walt, 20-Michael Leitch, 21-Fumiaki Tanaka, 22-Rikiya Matsuda, 23-Lomano Lemeki.