Announcers love the sophomore slump. It's easy to diagnose, so they throw out a few lines about the league adjusting to the player, and the player failing to make the proper adjustments in return. Case closed. There's no need to dig any deeper. They player just needs to adjust.
Middlebrooks also has the injury angle, since he broke his wrist last year. It's impossible to say how much this is affecting him, if at all, but it's an easy fall back.
A year ago Middlebrooks burst onto the scene in Boston, ripping everything in sight. The 23-year-old hit so well that the club's decision to trade Kevin Youkilis was an easy one. He cooled considerably in the latter half of the year, but still put up an impressive rookie season, hitting .288/.325/.509. The OBP was an obvious concern, but it's not as if Middlebrooks wasn't known for being a free swinger.
Last year he drew a walk every 22 plate appearances. This year that rate is even worse, with him getting a free pass every 26 plate appearances. This was a red flag last season, but not completely alarming. Now that's changed. The reason is that Middlebrooks can't hit. He's currently batting .193/.229/.379. He has the worst OPS on the team, and the worst among all AL third basemen with the exception of Chicago's Jeff Keppinger who is off to a historically awful start.
As a rookie Middlebrooks hit everything that wasn't over the middle of the plate. Here is his average based on zone.
It wasn't just middle-in pitches that he was ripping, but he was handling the outer third of the plate as well. Also note what he did with low strikes. On the lower third of the plate he hit .373.
Now compare that to this year:
In that same zone Middlebrooks is 3-15 (.200). The announcers keep talking about how pitchers are now pitching him away, and that's led to his sophomore slump. But he hit pitches away last year. The reality of what's happening this year is that he's no longer hitting pitches which are down in the zone.
Looking at the data it's easy to be an optimist in this case. It's not that pitchers are getting him out by finding his holes, but rather, that he's failing to hit pitches he was hitting last year. Chalk it up to the injury. Chalk it up to timing. Or stick with the announcers and just call it a sophomore slump.