Jack Conan's World Cup is over after Joe Schmidt confirmed he had broken his foot with Jordi Murphy expected to travel to Japan as cover.
The Leinster back row was supposed to start today's game against Japan but suffered the injury in yesterday's training run.
"Unfortunately Jack Conan has a fracture in his foot," said Schmidt after the 19-12 loss.
"He will return home to Ireland tomorrow."
Ulster's Murphy was due to start last night's Guinness Pro14 game against Ospreys but withdrew before kick-off and will now join up with the squad ahead of Thursday's game against Russia.
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Speaking to RTÉ Sport last week, Murphy said: "Unfortunately I fell just short of [selection].
"I'd never wish ill upon anyone but I just have to stay ready.
"It's an attritional physical game so I've to stay fit, which I'm doing with Ulster [but] if the phone call comes I'll be ready to go at a second's notice.
"Joe [Schmidt] said just keep the head down and keep working away and that's what I'm doing."
