Defensively speaking, how much would the hit be to switch from Drew and Bogaerts at shortstop and third, respectively, to Bogaerts and Middlebrooks? This is a tricky question to answer, because you're trying to project two very young, unpredictable players in Middlebrooks and Bogaerts. On top of that, Drew had a gruesome ankle injury at the end of 2010, which makes his 2011 numbers an outlier.
Bogaerts at short and third
In a comment on an earlier post, I was asked about what to make of the fact that Bogaerts was a plus defender at short and a well below average one at third. The simple answer here is not much. Bogaerts only logged 110 innings in the field and defensive stats don’t even stabilize in a single season, so his numbers overall aren’t meaningful, let along splitting them up by position. Scouts say that Bogaerts would be an about average shortstop, so the simplest thing would just be to give him a 0 for defense. At shortstop, that’s no small feat. At third, it becomes less clear. The rough estimate would be that he would be save 5 runs, per calculations by Tom Tango.
Drew's Defense, Looking Forward
For Drew, his runs saved per 150 games at short go like this for the last 4 years – 10.1, 8.4, -14.4, 6.7, with the terrible 2011 due to the aforementioned ankle injury. Drew certainly passed the eye test in 2013 and Baseball Reference pegs his defensive value at about the same (~6 runs), so I think it makes sense not to put any weight in the 2012 performance. Even excluding that year, Drew’s defensive value is on the downswing. A very, very basic regression puts his defensive value at about 5.5 runs next year and dropping 1.3 runs years thereafter. Steamer, a more sophisticated projection system, has Drew as a +6 run defender next year.
What to make of Middlebrooks?
Middlebrooks is also an unknown. He was slightly above average in 2012, but was a total mess in all aspects of his game in 2013. For his career, he has a UZR/150 of -3. Now, there were some reports that his struggles with hitting bled over into poor fielding, but we’ll stick with the numbers. Middlebrooks certainly looks the part of an above average third baseman, but he hasn't shown enough for us to label him as such.
Adding it all up
So, what does that all add up to? We can reasonably expect Drew and Bogaerts to be about 10 or 11 runs above average, defensively. Bogaerts and Middlebrooks would be slightly below average, at 3 runs below average. So for a single year, the Red Sox would probably be looking at giving up about a win and change, defensively. Of course, this doesn’t mean that resigning Drew is the way to go. The short term hit on defense would sting a bit, but long term, there are good reasons to think about letting Drew walk. Drew will be 31 on opening day, and his defense is already in decline. By the end of a four year deal, it doesn’t seem likely he’d still be a major defensive contributor. The $12 or $14 million could be well spent on finding an outfielder (Beltran? who could also be had on short years) or catcher. From the farm system perspective, the Sox are in good shape to replace him, thanks to Bogaerts, as well as other left side of the infield prospects such as third baseman Garin Cecchini who mashed at A+ and AA in 2013 and held his own in the pitching heavy Arizona Fall League (.772 OPS, 17BB:14 K). On top of that, the Red Sox would net a draft pick if Drew were to leave, thanks to him receiving a qualifying offer. All in all, it seems like a good opportunity to grab the draft pick and avoid a long term financial commitment that isn’t necessary.