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Lester's Annual Terrible, Horrible, No Good, Very Bad Month

It ranks up there with the great mysteries of our time: "How did they build the Great Pyramids at Giza?" "Was there really a second shooter in the grassy knoll?" "Who told Madonna that she could act?"

Yes, the great Red Sox mystery of the early 21st century: "Why does Lester have a bad month every year?"

It's gone from being amusing to becoming a very, very bad joke:

Month ERA that month Season ERA
August 2007 6.53 4.57
August 2008* 4.31 3.21
May 2009* 5.86 3.41
April 2010 4.71 3.25
May 2011 4.45 3.36

So what's underlying the periods of sucking like a Hoover in an otherwise stellar career?

Well now, here's where it starts to get interesting. Note the months shown: that April is Lester's worst month is actually a bit misleading. Only in 2010 was April actually Lester's roughest month ERA-wise. However, beyond that, it's hard to pinpoint exactly what's going on. For instance, it's not a tendency to give up the long ball, nor is it the result of excessive walks; in almost all of those seasons, there are months where he did just as well (or just as poorly) in terms of HR's and BB's, and still had significantly better results.

Could it just be bad luck? Let's take a look at a few more stats:

Month BAbip for month BAbip season
August 2007 0.302 0.280
August 2008* 0.297 0.300
May 2009* 0.361 0.314
April 2010 0.320 0.291
May 2011 0.350 0.298

Hmm. Now this is interesting. Except for August 2008, BAbip went way up during the bad months. Maybe time to look to see if he's the victim of bad fielding?

Month xFIP for month ERA Best monthly xFIP Worst monthly xFIP
August 2007 4.70 6.53 4.70 5.68
August 2008 4.04 4.34 2.88 5.19
May 2009 3.39 5.86 2.35 3.75
April 2010 3.45 4.71 2.49 3.91
May 2011 3.81 4.45 3.24 3.81

So only this year (for which we only have two months' worth of data so far) is the month in which Lester stinks up the joint the month when he actually pitches the worst, at least according to xFIP. However, looking at May 2009, we see an absolutely brutal disparity between xFIP and ERA—nearly two and a half runs! That's some pretty bad butchering. Admittedly, it's partly the fault of the pitcher: while Lester is known for many things, Gold Glove-caliber defense is not one of them. Moreover, he's admitted that he's had some rough outings here and there, such as the most recent stinker, in which only his curveball seemed to be working properly. However, when you have differentials of over a run, it's time to start pointing a few fingers at sloppy defense behind him—and that seems to be true more often than not.

With respect to the issue of there being a bad month every year, that does appear to be a bit of a construct we've created: in several years, he's had two relatively bad months, averaged out with some truly outstanding months, as well as a years where everything was about equal.

Finally, though, we're left with a classic chicken-or-egg scenario: is the defense sloppy because Lester is struggling, or is Lester struggling because the defense behind him gets sloppy? I don't have the statistics to back this up, but my personal sense is that Lester, moreso than most other Sox pitchers I've seen in recent years, struggles to regain composure after an error (particularly of his own commission).