However, a 19-3 victory in France saw Scarlets leapfrog Racing Metro into third spot and keep alive their hopes of claiming a place in the Challenge Cup.
Kristian Phillips scored the decisive try at the Stade Yves-du-Manoir with Priestland adding 14 points and the fly-half admitted it was a night when the result took priority over the performance.
"It was a poor game, to be honest. But we worked hard, even though sometimes we were our worst enemies by putting pressure on ourselves," he said.
"We wanted to simplify our game and our pack was outstanding. We didn't get the accuracy and continuity we expected but we showed great determination.
"We did well overall given that we have been in this situation in a few games this season and it hasn't always turned out well for us. We showed composure and managed to get that win. So that's a good thing for us."
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