Cardiff Blues head coach Danny Wilson has described Wales star Sam Warburton's latest injury setback as "a major blow for all concerned".
Warburton, British and Irish Lions captain on their last two tours, will miss Wales' autumn Tests this season.
The Blues said that former Wales skipper Warburton faces up to four months out following the recurrence of a neck problem that now requires surgery.
Warburton, who led the Lions in their drawn Test series against world champions New Zealand this summer, had yet to make his first appearance of the season after a post-Lions break before returning to training.
"It's a major blow for all concerned, it's a blow for Cardiff Blues and Wales. We all hoped he would be back playing soon," said Wilson, speaking at the Blues' weekly media session on Tuesday.
"It is a long-standing issue that he has had. He gets pain and discomfort on and off.
"The feeling was that two months off would help it settle completely and do him a world of good.
"He came back feeling confident about it. He went through the protocols when he came back to training, and during the contact element, just on pads, it began to flare up again.
"We looked into it in more detail, and it has led to where we are.
"He is obviously extremely frustrated. He thought that after two months he would come back really fresh, and that is not the case, but this is the right thing for him to do.
"He is a very good professional, and I am sure he will apply himself to his usual standards."
In a statement, the Blues said Warburton had "exacerbated the injury in training this week," adding: "He has undergone scans and has been reviewed by a consultant, who confirmed the management plan. It is likely that Sam will miss a period of up to four months."
Flanker Warburton has played 74 times for Wales and led the Lions in Australia (2013) and New Zealand. He was set to have been a key part of national head coach Warren Gatland's Test plans in November and December.
Wales launch their autumn series against Australia in Cardiff on November 11, before hosting Georgia, then the All Blacks and finally South Africa.
Ospreys and Lions flanker Justin Tipuric would be the clear favourite to wear Wales' number seven shirt in Warburton's absence.
The current recovery timescale, though, indicates that Warburton should be available for Wales' RBS 6 Nations campaign this season, which they begin against Scotland on February 3.
Warburton has undergone a handful of operations during his career, and the latest setback means he is denied an early rematch against New Zealand following the Lions' exploits.
A catalogue of previous injuries include a dislocated shoulder, torn hamstring, ankle ligament damage and fractured cheekbone, but he has always returned from such blows and maintained world-class form.
Warburton's loss from the autumn games could leave Gatland facing close assessment of his back-row resources.
While number eight Taulupe Faletau has been performing superbly for Bath this season, Gloucester's Ross Moriarty has yet to return from injury and seasoned Ospreys campaigner Dan Lydiate has been recovering from knee ligament damage suffered late last year.
Openside flanker specialist Warburton has also filled the blindside role on a number of occasions for Wales, but that option has now been lost to Gatland as he looks ahead to the autumn fixtures.