The Scarlets outside-half was helped off with a knee injury seven minutes from the end of the Scarlets’ 27-15 RaboDirect Pro12 victory over Cardiff Blues on the final day of the regular season.
“It looked like a medial ligament injury, but it’s difficult to assess the severity of it at the moment,” said Scarlets head coach Simon Easterby.
“It was disappointing because Rhys was playing really well. He had set up a couple of tries and just looked like he was enjoying himself out there.
“It is disappointing it happened so close to the end and it could possibly hamper his chances of making the summer tour.”
Priestland began this year’s Six Nations as Gatland’s primary 10 and was largely disappointing before being dropped for the final game against Scotland with Ospreys star Dan Biggar starting in his place.
The injury could settle the debate over who should be first-choice 10 for Wales with Biggar topping the scoring charts in the Pro12 again this season.
Leicester Tigers outside-half Owen Williams could also be a beneficiary from his former team-mate’s demise after a superb season with the Aviva Premiership side despite his inexperience on the international stage, while Perpignan’s James Hook may finally be able to make a case for the 10 jersey despite being out of favour with Gatland.
Gatland already has an injury crisis with captain Sam Warburton, full-back Leigh Halfpenny, hookers Richard Hibbard and Emyr Phillips, flankers Justin Tipuric and James King and centre Ashley Beck all out for the summer.
MORE: The latest football news
MORE: The latest tennis news
BIOGRAPHY: Daniel Sturridge
BIOGRAPHY: Kepa Arrizabalaga