Newport Gwent Dragons centre Jack Dixon is waiting for results on a possible kidney injury suffered in a pre-season friendly.
The talented young back's condition is being reviewed this week but he could be out for as long as six months.
Dixon, a former Wales U20 international, trained with the senior Welsh squad earlier on in their Rugby World Cup preparations, taking part in their overseas training camps in Switzerland and Qatar.
"Jack Dixon suffered a blow to his kidney region during a friendly against the Scarlets on Friday," read a short statement.
"The 20-year-old is currently being reviewed by a specialist.
"The region will provide further information in due course."
Dragons head coach Lyn Jones additionally told the BBC: "It's very sad. He's made good progress in his short career and he's trained well with Wales but unfortunately didn't make the cut.
"We were expecting big things from Jack but unfortunately his development has again been curtailed with a nasty injury to his kidney, which is going to keep him out for three to six months.
"We'll have to assess his recovery come November."