The Wales star ruptured his left Achilles tendon during the Scarlets' European defeat to Exeter Chiefs on Saturday.
He underwent corrective surgery today and faces at least six months on the sidelines, realistically ending his season.
"It's a big blow for Rhys and for our region to lose a player of his quality and influence in our team," said Scarlets head coach Simon Easterby, a Lions tourist in 2005.
"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 in our game against Exeter - so it's very difficult for him to take; but he'll 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 will be back and can be a very positive influence in our environment.
"He's a committed Scarlet and 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 games during rest of the season.
"We all want to show him as much support as we can right now, it's going to be a tough few weeks for him to accept what's happened but the boys here are a very close-knit group so I'm sure that will be a positive for him."
Priestland is just the latest casualty for Wales ahead of their Six Nations defence.
Alun Wyn Jones, Dan Lydiate and Ian Evans are all set to miss the start of the Championship, which kicks off when Wales welcome Ireland to the Millennium Stadium on February 2.
Lions trio Adam Jones, Jamie Roberts and Leigh Halfpenny are also currently sidelined.