Gordon D'Arcy's stomach bug is the "sole" reason for in-form Luke Marshall starting Ireland's RBS 6 Nations opener against Scotland, according to head coach Joe Schmidt.
And Schmidt has hailed Ulster centre Marshall's Heineken Cup form as the deciding factor in giving the 22-year-old the nod to step in for D'Arcy on Sunday.
Marshall helped Ulster complete their first-ever 100 per cent group-stage record in the Heineken Cup, including doing the double over Leicester and winning in Montpellier.
Stalwart centre D'Arcy, 33, returned to full training after suffering a stomach upset at the start of the week, but was off the pace.
The 75-cap centre will therefore not get the chance on Sunady to extend his glittering, long-term partnership with Brian O'Driscoll, who will win a record 129th cap against the Scots.
But former New Zealand schoolteacher Schmidt backed four-cap Marshall to shine this weekend, especially when flanked by Johnny Sexton and O'Driscoll in Ireland's back line.
"Gordon's stomach bug was the sole selection criteria, really," Schmidt said. "Luke Marshall has played really well in the Heineken Cup in big games.
"He played in the Six Nations last year and played very well against Scotland.
"I thought he played well against Australia in the autumn too. He made a defensive error at one stage, but he's learning all the time. He's working hard at everything we're trying to do.
"He's between Johnny Sexton and Brian O'Driscoll, so experience-wise, he's a bit inexperienced but either side of him he has two players who can help him navigate his way through the game."
Scotland centres Alex Dunbar and Duncan Taylor have just nine caps between them, but Schmidt said that played no part in promoting Marshall.
Despite D'Arcy's illness, Schmidt admitted he still nearly pitched the Leinster centre into action on the opening Six Nations weekend.
"It was a very, very close call," Schmidt said.
"Because it was a tight call, Luke got a little bit more of the training time and would feel a little bit more comfortable.
"Gordon played as well as I've seen him against the All Blacks in the autumn, so he will be important for us across the tournament.
Isaac Boss takes a seat on Ireland's bench for Sunday's Scotland showdown, with Eoin Reddan ruled out through calf trouble.
Wing Luke Fitzgerald is battling an ongoing hip complaint, paving the way for Andrew Trimble to make his first international appearance since last summer.
Backing Ulsterman Trimble to seize his chance, especially against Scotland's hefty wing duo Sean Maitland and Sean Lamont, Schmidt admitted he does not know how long Reddan and Fitzgerald would be sidelined.
"Eoin and Luke are both in the same basket," he said.
"I guess we won't know until next week whether either will be right for the second round, against Wales.
"The one thing we felt with Luke was that if we tried to push too hard this week we might lose him for more than next week anyway.
"Andrew brings a bit of size, up against Lamont and Maitland.
"We just don't have that many sizeable men, barring dragging Shaggy [Shane Horgan] down from the commentary box!
"We don't have too many men of that size and stature, so Andrew is a good contrast for us.
"And, to be fair to Andrew, he's played very well in the last couple of games, and from that respect he deserves an opportunity."