Largest slice of blame pie - the coaching staff
Ben went out and signed the top 2 position players in the free agent crop. You would have been a fool to expect sunshine and rainbows from them, but they were expected to be helpful, and Ben got them both on pretty reasonable deals by 2015 standards. (ignore the money - functionally doesn't matter, but the years were totally acceptable) Dustin Pedroia is healthy. We are not relying excessively on old farts - this has been a large, systemic level of offensive underachievement (Bogaerts and Pedroia aside).
He let Lester go and peopled the rotation with a bunch of mid-rotation sort of guys. Aside from Buchholz they have not pitched like mid-rotation pitchers. Porcello has pitched more like back end, and Miley and Kelly have pitched like guys who might not be actual big league starters. For a bunch of guys in their 20s with either stuff or track record to be *decent*, this is serious underachievement.
When the team - with talented players, who have actually not shown that much evidence of non-talent, all fail together, that points to a systemic flaw. I would be all for a managerial change here - something needs to be shaken up.