Priestland, 25, will miss the entire RBS 6 Nations Championship after snapping his Achilles tendon during the Scarlets' Heineken Cup clash against Exeter at the weekend.
Priestland, who has 22 caps, also looks set to miss out on the British and Lions' tour of Australia next summer.
Scarlets head coach Simon Easterby said: "It's a big blow for Rhys and for our region to lose a player of his quality and influence in our team.
"He was coming through well in the last couple of (Wales) games of the autumn, put in a strong performance against Australia and was making a real impact on our game against Exeter, so it is very difficult for him to take.
"But he will get plenty of support from here throughout the injury period."
Scarlets medical chief Andy Walker added: "The surgery was undertaken this morning (Tuesday) at the Vale Hospital and has gone very well.
"It's 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.
"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.
"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."