The Scarlets fly-half's hopes of touring Australia with the British and Irish Lions next summer also appear to be over after he suffered a ruptured Achilles tendon.
The 25-year-old, who was hurt in last weekend's Heineken Cup game against Exeter, underwent surgery today and faces six months out of action.
It has left Wales nursing another major injury headache less than two months before they launch their Six Nations title defence.
The casualty count greeting interim head coach Rob Howley currently runs into double figures.
Howley, in charge for the Six Nations while Warren Gatland prepares full-time to head up the Lions tour, faces being without Priestland, prop Aaron Jarvis, lock Alun-Wyn Jones and flanker Dan Lydiate when Wales tackle opening opponents Ireland and France.
Full-back Leigh Halfpenny, centre Jamie Roberts and lock Ian Evans, meanwhile, are unlikely to make their playing returns until shortly before the tournament starts, while the likes of George North, Richard Hibbard and Adam Jones have also had injury issues.
With Priestland sidelined, Howley would appear to have a straight choice between Perpignan's James Hook and Ospreys fly-half Dan Biggar for number 10 duties.
And Biggar's Ospreys understudy, 20-year-old uncapped prospect Matthew Morgan, could now find himself in the Wales Six Nations squad as a further fly-half option.
In the meantime, all thoughts are with Priestland as he begins a long recovery process.
"It's a big blow for Rhys and for our region to lose a player of his quality and influence in our team," Scarlets head coach Simon Easterby said.
"He was coming through well in the last couple of games of the autumn, put in a strong performance against Australia and was making a real impact on our game against Exeter, so it's very difficult for him to take.
"But he will get plenty of support from here throughout the injury period.
"I am sure that a player of his experience, intelligence and ability will deal with this very well, and after a couple of weeks away to recover from the surgery he will be back and can be a very positive influence in our environment.
"He's a player with a very good rugby brain, and there will be a lot he can do to work in and around our environment to help the team, some of our younger players and in our preparations for the rest of the season.
"We all want to show him as much support as we can right now. It's going to be tough to take, but the boys here are a very close-knit group so I'm sure that will be a positive for him."
Priestland, who has won 22 Test caps, was a driving force behind Wales' outstanding push to reach the World Cup semi-finals in New Zealand last year.
He then featured prominently during last season's Six Nations, when Wales were crowned Grand Slam champions, and although the latter half of 2012 saw him struggle to reproduce that form he remained a key member of the Wales team, starting three of the recent autumn Tests and going on as a substitute against Samoa.
Scarlets medical chief Andy Walker revealed that Priestland's operation had been performed using a new keyhole technique.
"Rhys' surgery was performed by a new keyhole surgery technique which is non-invasive and will aid his recovery and allow him to get into rehab with our medical team at a quicker stage," Walker said.
"The anticipated time off the field is around six months, but it all depends on the individual and how his rehab goes in the interim.
"The surgery has gone very well. It is a difficult injury, but we have been fortunate to have had the injury assessed and repaired surgically so promptly, and by a very experienced ankle and foot specialist.
"He will be non-weight bearing for two weeks now, in plaster and then into a cast boot, and then we will start rehab at a later stage.
"This is a serious injury, and the most important thing is that the medics do everything possible to ensure a strong recovery and not to rush the process at any stage."
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