Not actually convinced that Quintana has regressed per se. I look at his numbers and see a pitcher who's getting bitten by a quest to pursue the three true outcomes. All of Walks, Strikeouts and Homers have spiked for Quintana, while at the same time, BABIP is more or less within career norms..
That tells me that Jose Quintana does not trust his defense, or else is trying too hard to carry the team beyond his ability to do so, and is deviating from his approach by not pitching as much to contact, trying to overpower hitters and get them out himself rather than trusting the team around him, with predictable results. Similar to some of the nonsense Josh Beckett pulled over the years when the team started going bad and he'd try to get everyone out himself. Like Beckett back then, what you saw was an increase in walks, strikeouts, and homers whenever he did that.
Put him on a team with a dependable defense, with other reliable starters, a team that was going somewhere and he didn't feel he had to carry the whole load himself, and I think you could see a big improvement in numbers once he shakes the bad habits he's picked up this year and learns to trust his D. I think the numbers are telling me that Q is uptight, feels too much pressure on his shoulders and not satisfied with his team right now. That said I do think he may be traded in the near future if the numbers read true.