Well ... let's take a look at run attribution from a different slant.
It is possible to get a qualitative look at defensive run attribution by analyzing hits allowed (1B, 3B, 3B and HR) against hit trajectory (GB, FB, LD). The general notion is that by looking at, for instance, the team's 1B/GB rate against the league average, we can get a rough estimate of the singles that could be attributed to the team defense.
First, though, we need to equalize the numbers a bit, since the Sox pitching staff gave up GB, FB and LD rates quite different from the league average. In fact, the Sox pitchers gave up fewer GB's per BIP than the league average (41.5% to 43.8% league average), more fly balls (38.8 vs 35.9) and roughly the same number of line drives (18.9 vs 19.2). In total the Sox pitchers gave up 44 fewer BIPs than the league average, so in general, the Sox staff did a pretty decent job overall last season.
To equalize the rates, then, we calculate the 1B, 2B, 3B and HR per BIP rates, then multiply them by the league BIP average to yield something resembling a BABIP-neutral assessment of hits allowed for each of the hit trajectory. Here are the results for the 2009 Red Sox compared to the league average:
1B 2B 3B HR
GB +21 -6 -1 NA
FB +24 +23 -2 -7
LD +18 +1 -2 -4
Tot +63 +18 -5 -11
Looking at ground balls first, I think that it is safe to assume that the Sox 2009 infield was a bit on the pourous side, allowing 21 more singles than the league average. There are no real mitigating factors other than karma to adjust this rate, and it is probably safe to assume that we can assign all of these extra singles to shoddy defense, or roughly 10 runs.
I think that the six fewer doubles on ground balls can be largely attributed to the dimensions of Fenway. Hot grounders hit to the right or left field corners would account for nearly any extra base hit on a ground ball, but the Monster probably prevents many of these hot grounders down the left field line from becoming doubles. In general, however, I would find it difficult to penalize a fielder too badly for a ball hit hard down the line.
Fly ball analysis is a bit trickier. The Sox allowed +24 more singles on FBs than the league average, and to get a better picture of the skew, the numbers were 101 against Boston vs 77 against the league average, so the skew would probably be statistically higher if the Fenway numbers were deducted from the overall set. There are essentially two types of FBs that land for singles - seeing eye pop flies and balls hit off the Monster. I suppose that a very small percentage of these could be attributed to a fielder, but most are clearly karmic.
Similarly, the +23 doubles on fly balls were probably primarily Monster shots.
Line drive analysis is a little trickier. We should logically expect that the Monster will increase singles to some extent, and probably reduce doubles (unlike fly balls) by the same general rate (thus converting balls that would be doubles in a bigger park into wall-ball singles). I think that there are potentially more subtle effects too. A left-fielder in Fenway generally plays much shallower than in other parks. If positioned correctly, the left fielder may be able to turn an occasional potential line drive single into an out, but I also think that playing shallower limits the outfielders range to his left or right, so may cause some potentially catchable balls to go to the wall. I also think that the larger-than-average zones in CF and RF may cause a few extra line drive singles, but may reduce some doubles and triples if the fielders are deep. Conversely, they could also go the other way if the fielders are cheating (either CF toward the Monster or RF toward the corner). Perhaps the most interesting scenario would be a LF shaded toward the Monster against a righty power hitter. Many of the great righty hitters have ample power to right-center, especially if pitched outside (as most are). This duality probably increases the difficulty of playing CF in Fenway.
Much of this is speculation, of course, and I'll try to take some time to look at past seasons to see if any solid trends emerge.
The bottom line, however, is that I don't think that either UZR or FB ratings accurately account for the idiosyncracies of fielding in the Fenway outfields. I believe that Dewan has made some adjustments on fly balls, but I don't believe he has done anything with CF or RF zones. I have no idea about UZR, but just a glance at the numbers that EV provided would seem to indicate that UZR doesn't correct as much as Dewan does.
I think it's a much fairer assessment of the 2009 team to suggest that the defense accounted for maybe 20 or 25 runs below the league average. If you give the defense full penalty for all of the +64 1Bs and +18 2Bs in the chart above, that still only accounts for 37 runs (using standard LWTS values). I also think that adding the new fielders will help the infield, perhaps to the tune of a 20-run swing, but will have much less effect on the outfield defense than predicted. Even the infield numbers may be smaller, given the fact that the Sox pitchers allowed 120 fewer groundballs than the league average last season, and adding Lackey is likely to increase that gap. My general rule of thumb with UZR and FB fielding runs is that if you cut them in half, you're probably in the ballpark, so we may see a 40-run swing, probably less.