Joe Schmidt insists Simon Zebo must feature at full-back against England to be ready to deputise for Rob Kearney at the World Cup.
Ireland boss Schmidt has opted to field Zebo at full-back ahead of regular starter Kearney in Saturday's final World Cup warm-up at Twickenham.
Kearney has suffered a bruised knee but trained fully and was available for selection for the England clash.
"We just felt it was important to get Simon a little bit more time at full-back once we decided he was going to cover full-back for us," said Schmidt.
"Rob Kearney has a bruise on his knee but he trained fully and we could have picked him."
Ireland have wheeled out the big guns to face England, all barring Kearney and Cian Healy, neither frontline star risked with the World Cup opener against Canada on September 19 hurtling into view.
Healy has finally returned to full training after battling to recover from neck surgery in May, but Schmidt has handed the loosehead prop extra time to hit top speed.
Ireland's Kiwi boss believes Keith Earls would have been ready for action had he been required this weekend, despite suffering concussion in last Saturday's 16-10 Dublin defeat to Wales.
Luke Fitzgerald was also available for selection, but Schmidt opted instead to hand Ulster's Darren Cave the final seat on the bench for the England trip.
"Keith (Earls) has completed his return to play protocols thus far, so we'd expect that he would probably have been fit to be selected," said Schmidt.
"But the thing with Keith and what we're trying to do with balance at the moment, there are some guys that play really well fresh and others that need to get a bit more a game rhythm, and Keith's had a couple of outings.
"In the first game in the Millennium I thought he looked super-sharp, so to have him fresh where there's a small degree of doubt then it just made sense.
"Luke (Fitzgerald) trained fully through the week, did a little bit of contact today, no problem. He would be fully-fit, but he's had two starts already.
"We wanted to give Simon another run at full-back, and Dave Kearney and Tommy Bowe again on the wings.
"We thought about putting maybe Luke or Rob Kearney on the bench, but Darren Cave is the other guy who has had the least amount of game-time so it was good to get Darren into the group as well.
"Because with Jared (Payne) and Robbie (Henshaw) we can always drop someone else back to full-back, they've slotted in pretty seamlessly in amongst it at training this week."
Despite reuniting first-choice lock combination Paul O'Connell and Devin Toner, Schmidt admitted Ulster's Iain Henderson and Donnacha Ryan are pushing the engine-room argument to the limit.
"Iain's had two full games so far, Dev has had a cameo really by comparison," said Schmidt.
"I did probably try to muscle the skipper out to be honest, but he wasn't for budging!
"He felt he needed another game.
"It was good to get that combination back together again with Dev and Paul, they've served us really well together in the last two Six Nations.
"And Iain has given us the right sort of headache, without a doubt.
"It wasn't a form selection: it was a balance of game time and opportunity, so that we've got as many guys as ready to hit the ground running as possible in two weeks' time."