He did have a clean second innings, so it might have been rust (also only had my eye on it a bit). I'm not _worried_ about Priester, but I do think it's worth pumping the brakes on him in terms of being a dominant SP. It depends a little bit on definitions, but I think what's most appealing about him is that he should project well to be an innings anchor and above average. Without some changes to his fastball shape, I don't see a dominant pitcher, but it's not out of the question that he could find these changes in AA this year.
The problem is that the strength of his other pitches would play better off a better fastball, in addition to the fastball just generally still being the most important pitch. We'll see, I think he's too young to really declare anything too definitively, but the way it's trending is that his fastball won't be any better than average, and that's not a recipe for a genuine impact pitcher. That said, a stable #3 type who gives you a chance to win every game is still an extremely valuable (and increasingly rare) player.