Wales prop Samson Lee has only a "50-50 chance" of making the World Cup in September after rupturing an Achilles tendon.
Head coach Warren Gatland said the 22-year-old Scarlets tight-head will need an operation on the injury.
Lee suffered in the injury in the 12th minute of Wales' 23-16 Six Nations win over Ireland.
"It is a longer-termer that could potentially keep him out of the World Cup," said Gatland.
The New Zealander said Lee had been due to have an operation on Monday, but the procedure was cancelled because there was too much swelling around the injury.
The surgery is now likely to take place next week, with Gatland adding: "I think he has a 50-50 chance of making the World Cup.
"We have two warm up games before the squad is announced in the summer, if he's up and running and things go well he won't be too far away.
"Do we take the risk and name him without too much rugby? In the end it will be up to the medics."
Wales kick off their World Cup campaign against Uruguay on 20 September, and are in the same qualifying pool as hosts England and twice former winners Australia.
The injury forced Gatland into making one of two enforced changes to his team for next Saturday's Six Nations match against Italy in Rome, with Aaron Jarvis taking the number three shirt.
The other change sees Scarlets loose-head Rob Evans called in for his first international start in place of the 114-times-capped Gethin Jenkins who has a hamstring injury.