Jack Conan will fly home from the Rugby World Cup on Sunday due to a fractured foot, leaving Ireland to find a response to their shock Japan defeat without the back-rower.

Conan suffered the injury in training and missed Saturday's match against Japan, with Joe Schmidt's side sensationally beaten by the tournament hosts.

Schmidt, speaking after the 19-12 loss, confirmed Conan would be leaving the squad, with reports suggesting Jordi Murphy will be called up as his replacement.

"Unfortunately, Jack Conan has a fracture in his foot and he will return home to Ireland tomorrow," Schmidt said.

Schmidt was hopeful there had been no further injury issues against the Brave Blossoms, but Rob Kearney took a blow to the head and failed his initial assessment.

"From an injury perspective, Rob Kearney, I just spoke to him, has got a decent shiner but he's feeling okay now," he said.

"He failed his HIA [Head Injury Assessment], as far as I know, so if he doesn't pass his HIA 2 and HIA 3, he'll be a confirmed concussion and that will rule him out of the Russia game."

The coach is now focused on the team bouncing back against Russia, although he is wary of a short turnaround to the match on Thursday.

"It's a tough defeat for us to take, but I think the strength of the team will be how they respond and rebound from this," he said.

"A six-day turnaround into this game was a little bit tough, and we've got a five-day turnaround into the Russia game in Kobe on Thursday."