Conan was set to start in Shizuoka before being accidentally stood on by a team-mate during Thursday's training.

"At this stage, it's just a [case of] wait and see," said Feek.

"Obviously we are going to get an opinion on that in the next couple of days and then go from there."

Leinster's Conan came on as a replacement in last weekend's dominant win over Scotland and is seen as a key player within Joe Schmidt's squad having enjoyed an impressive season with the Pro14 champions.

On Friday, Feek was keen to downplay the idea that Conan's injury could have tournament-ending consequences.

"It is a little bit of a set-back at this stage," he said.

"But I think once we get the game out of the way and get the reports in from medical and he gets assessed fully, we will probably have some more information on that."

Peter O'Mahony will start at blind-side flanker having come through concussion protocols after been taken off in the first half against Scotland, while CJ Stander remains at the back of the scrum following his man of the match display in Yokohama.

The outcome of Ireland's meeting with Japan will dictate who takes control of Pool A, with both sides going into Saturday's encounter having secured bonus-point wins in their opening matches.