Was it luck or a somewhat probable outcome that the FO planned poorly for? Middlebrooks already had a hamstring issue earlier in the season and those tend to linger so this certainly wasn't an out of the blue injury.
The bigger issue is that while WMB was outperforming Youk he was a little dinged up and likely Adrian is also more than a little nicked up/tired/both. Between the two of them it should have been rather easy to rotate 3 position players. If anything it probably would have been beneficial to rest Adrian a bit, especially after forcing him into RF. Why Bobby V pencils him in day in, day out is a little puzzling.
There's plenty of blame to go around, but this strikes me as the most salient issue: WMB was injured earlier, ran awkwardly and slowly for a few games, is still a little bit clumsy on defense, and benefited from a few games on the bench. At this point, Middlebrooks is still a mistake hitter, not a phenom, and he will continue to struggle until his pitch recognition improves on low changeups. Youkilis was not hitting yet since returning from the DL, but he was looking much better on defense at both 1B and 3B. With Ortiz, Gonzalez, Youkilis and Middlebrooks sharing 3.5 positions (counting Gonzalez as a fraction of a rightfielder), there should have been enough at-bats to go around. Instead, Cherington held a Youkilis fire sale, and got a fire sale return.
I blame this on Bobby Valentine for mismanaging Youkilis from spring training onward. It is the manager's job to get along with his players as best he can, until the GM can get him new players.
I blame this on Ben Cherington for humoring his egocentric manager and tipping his hand to the entire league. Cherington's trades specialize in value subtraction.
Anybody else within reach, I'll blame them too.