Priestland picked up a foot injury in training last week and was forced off at The Twickenham Stoop after the problem flared up late on in the three-try win over Quins.
The potential loss of one of their most impressive performers would be a big blow to coach Simon Easterby, especially as the 26-year-old appeared to be really hitting his straps after a lengthy lay off with an Achilles injury.
"It would be a blow to lose him because he was a catalyst for many of our tries," said Easterby, who was tipping the Scarlets trio of Priestland, Scott Williams and Jonathan Davies to shine for Wales in the Dove Men Series after his side's victory in the English capital.
"That combination of 10, 12 and 13 working for us is certainly something that the Welsh team will see as a positive. Hopefully there is a window of opportunity with the Welsh team.
"I'm happy that they got their rewards against Quins and hopefully that will catapult them on to play for Wales in the autumn. It would be a seamless transition for them."
Easterby also has concerns over winger Kristian Phillips, who is suffering with a calf strain, while Ken Owens and Rob McCusker are already out of the Round 2 fixture having also been unavailable at Quins.