The Irish camp insist Joey Carbery is fully ready to deputise at scrum-half after "numerous occasions" honing the role in training.
Fly-half Carbery will cover scrum-half from the bench in Thursday's World Cup clash with Russia in Kobe. Luke McGrath starts at nine in place of Conor Murray, who is being rested.
Ireland gambled on selecting just two recognised scrum-halves in their 31-man squad to bolster the stocks in other positions, with Munster star Carbery stepping up to cover both nine and 10.
Skills coach Richie Murphy revealed Carbery long since declared himself comfortable covering scrum-half, with Ireland then working hard to bring the 23-year-old up to speed with the unfamiliar task.
"It's something we've obviously been thinking about for a long time, back home," said Murphy, of Carbery's scrum-half duties.
"The unfortunate thing for Joey was the amount of time that was taken off him after the ankle injury. But we still felt it was a way we were going to go.
"And once we've come over he's done some work on that. He's trained there on numerous occasions.
"He feels comfortable. And we didn't just throw him in there, we asked him how he felt about it.
"And we made a decision in relation to the squad of players that we brought, about only bringing five half-backs.
"And his versatility in relation to playing 15, playing 10 and being able to cover nine was one of those factors.
"So he's happy to do it, he'd obviously prefer to be playing at 10 but these are the days where you put the squad first, and in fairness to Joey he's done that."
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Ireland's shock defeat to hosts Japan has opened up Pool A, but Joe Schmidt's men can still fight their to the quarter-finals with victories over Russia and Samoa.
Schmidt's side let a 12-3 advantage slip in defeat by the Brave Blossoms, but assistant coach Murphy insisted it would be over the top to brand Japan more skilful than Ireland.
Asked for his view on criticism that Ireland had slipped behind Japan on the skill front, Murphy said: "I think it probably is an unfair observation of our team.
"I sat here last week and everyone was saying how well we'd done the week before. So we can't go from one week to the next and become a really bad team overnight.
"So I think the Japanese were very good last week, they played at an incredible level of intensity and speed and were very accurate in their passing.
"We trained against Kobe Steelers today, and most of their players are in their early 20s, and their skill levels were very high as well.
"So it's obviously something that they work very hard on here. But it's something that we would be happy with at home."
Follow Ireland v Russia on Thursday (kick-off 11.15am) via the live blog on RTÉ.ie/Sport and the News Now App, watch live on RTÉ2 from 10am or listen to live match commentary on RTÉ Radio 1 from 11am.