First - he moved from a hitters park to another hitters park. Fenway gives up a lot of doubles, more than Comerica but fewer homeruns. Both are in the Top 10 in Park Factors, neither are Coors level outliers. The Tigers were also one of the league's worst defenses last year and the Red Sox were one of the league's best. That figures in also.
Second, and this is the key point in the Porcello discussion. Instead of looking at the ceiling, which is debatable, look at the floor, which is not. He did not fail to be a top of the rotation starter, or fail to be better than the magical 4.00 ERA or whatever goalpost is used. He failed to be what he was last year, the year before, or years before that. He was a truly bad pitcher as a 26 year old, which defies any sort of non-injury related explanation. Now this happens very occasionally (since I saw Rick Ankiel as a pitcher, and there are no absolutes), but the odds are strongly against that being the future level of performance. Without the revelation of an injury that bothered him all year, I am much more comfortable looking to the coaching for an answer to him becoming below replacement level than a 26 year old suddenly aging at a twentyfold rate.
When I am critical of the coaching, it is not because of one guy - it is because of the sheer number of guys who underperformed career norms across all aspects of baseball. And it's not like they are a team brimming with old folks. This was actually the sort of case which you almost expected a managerial change midseason.