Kenki Fukuoka almost didn't feature in Japan's shock win over Ireland at the Rugby World Cup on Saturday, but his last-minute addition to the squad made all the difference.

The winger's 58th-minute try gave Japan the lead in their 19-12 victory at an electrified Shizuoka Stadium Ecopa and ensured the Brighton Miracle of four years ago would not go down in the history books as a fluke.

"I knew (Ireland) were higher ranked than us, but I just told myself to go for it and that led to this good result," Fukuoka said.

"(The atmosphere at the venue) was amazing. The cheering was what allowed us to keep pushing until the end."

Fukuoka had initially been left off the 23-man squad to keep him well-rested for next week's Pool A clash against Samoa, but a hamstring injury to William Tupou in the morning forced head coach Jamie Joseph to move Lomano Lava Lemeki to the left wing and bring Fukuoka onto the bench.

The 27-year-old Panasonic Wild Knights talent said he had a bad case of pregame jitters, but it did not show as he replaced Ryohei Yamanaka in the 49th minute and quickly justified his inclusion by scoring off a perfectly timed offload from Timothy Lafaele.

"We had a plan to use (Fukuoka) against Samoa and we accelerated that a bit," Joseph said.

"I remember his first touch and he took it forward and broke away. If he'd played a bit more rugby, I think he would have scored (more)."

While the win brought back memories of Japan's historic upset of South Africa at the 2015 World Cup in England, as well as placing them on top of a pool that also includes Scotland and Russia, Fukuoka said there was no time to bask in glory.

"We play again next week. This was a really big win for us, but we need to be looking to the next match now," he said.