Ireland can effectively give themselves three weeks to prepare for a likely Rugby World Cup quarter-final against South Africa with a win over Japan on Saturday as Coach Joe Schmidt has backed his third-choice out half to help clear the way.

With Scotland comfortably dispatched and Russia and Samoa to come, Schmidt chose to rely on 27-year-old Jack Carty and give him his second test start.

Schmidt chose not to risk to play Johnny Sexton – who did not train fully this week – or rush Joey Carbery  back into the number 10 jersey after injury.

Making four changes from Sunday’s 27-3 win, Schmidt said he wanted to keep players fresh and look at different combinations, having learned a lesson from the last World Cup when he had to take on Argentina in the last eight with three players who had not started a big pool game and another out of position.

“We always knew we were going to have to manage this start to the tournament because of a six and a five-day turnaround, and we do know that the challenge this weekend is going to be particularly tough,” Schmidt said, referring to the gap between the Scotland and Japan games, and the next against Russia.

“We have got to show confidence in the squad because we have that confidence. In the last World Cup there were some players that didn’t play a lot and then suddenly they were required for the quarter-final and maybe they didn’t quite have the confidence we would have liked them to have.”

Ireland are ready for their big travelling contingent of fans to be in the minority in the Shizuoka Stadium after playing what Schmidt described as a virtual home match in Yokohama last week in front of a sea of green.

Japan’s players have admitted that nerves got the better of them before they ran out 30-10 bonus-point winners over Russia in the tournament’s curtain raiser and have made five changes, most notably with captain Michael Leitch dropped to the bench to make room for the more explosive Amanaki Mafi.

Coach Jamie Joseph said all 23 will require a performance to match the famous 34-32 win over twice world champions South Africa in 2015 if they are to cause another World Cup shock and open up Pool A.

“Our goal is that we want to do something that’s never been done in Japan before and that’s top eight. That’s what we’re about. That will leave a legacy for rugby in Japan, undoubtedly,” Joseph said.

“We are re focusing on us and making sure they are prepared physically, mentally and tactically to play the best game of their lives because that’s what they are going to need to do if we are going to be successful on Saturday”.