Dear Mr. Moscoso,

I have been in the habit of saying bad things about you on Twitter. To put it most baldly, I have expressed on multiple occasions that I do not think you are a very good pitcher. I have noted the straightness of your fastball and the general inadequacy of your breaking pitch. I have acknowledge that your changeup is pretty solid, I think, but in the same breath, I probably pointed out that your strikeout rate is a pitiful 4.6 per nine innings.

As to how you've managed a 2.51 ERA, I point to your absurd .200 BABIP and weirdly low 6.4% HR/FB rate (weirdly because of your extraordinary fly-ball rate, 24.6% ground balls according to Fangraphs, and probably still weird even considering your home park). What this adds up to is a mediocre 4.69 FIP or a 5.29 xFIP. That xFIP is double your ERA!

And yet, I watched you tonight (even though the game was this afternoon -- I was at work, and I have MLB.tv for a reason) pitching against (granted) the Seattle Mariners (and pitching well, again, going seven innings without allowing a run or even really being threatened for a run, making the pitiful two runs the A's eked out entirely on the back of Scott Sizemore, who homered and singled in a run, against Jason Vargas, the epitome of "soft-tossing lefty", stand up for a victory) and I realized something. I realized that, Jesus lord gracious, you induce a lot of weak fly balls and pop ups. Even before this game, in which the Mariners hit a ton of pop ups, Fangraphs says that 15.4% of your fly balls have been caught by infielders.¹ That certainly sounds like a lot!

I'm not entirely sure where to find pop up rate so I can know the league average, but Tommy Bennett said in this piece at Baseball Prospectus last year that the league average was in the 7-8% range. So if I'm doing my math right, this means that my eyes are not lying, because you are getting ... hm, carry the one ... a giant assload of pop ups!

Unfortunately, that same Bennett article is inconclusive on the question of whether pop up rate is an individual skill such that we'd expect individuals at the major-league level to have a wide variety of rates depending on their individual repertoires and approaches to pitching. This cursory analysis by David Appelman at Fangraphs concludes that pop up skill is likely not an independent skill but merely tracks a pitcher's ground-ball rate (non-linearly, it should be noted). You do have an extreme ground-ball rate, as I noted above. But if I extrapolate the curve on the Pop% vs. GB% graph in the Appelman article, it appears that to arrive at 15% in Pop%, you would need ... hm, yeah, looks like a 0% ground-ball rate.²

Now look, Guillermo. You seem like a swell guy. You get pretty fired up when you strike someone out in a key spot, which is pretty cool on this particular team because outside of Grant Balfour, I can't think of a single A's pitcher who ever expresses emotion on the mound. Not that I'm for such expressions in any meaningful way, but I'm not against them either, and I like a little variety of personality on the team I root for. You've provided that variety this year. Anyway, back to the point, your results so far are undeniable -- you've cracked fifty innings in the majors this year and you've allowed 17 total runs. That's pretty good! Nobody can take that away from you.

I say all that because I'm about to get back to where we started: what the A's can take from you, and what the A's probably should take from you, is your rotation spot as soon as someone better (like Tyson Ross) is available to do so. Just like you should bank your performance so far and never let it go, so should the A's. I like you, Guillermo, but I simply cannot advocate for your future in the rotation given the likelihood that you're in for some serious regression while your potential replacement, Ross, is a youngster with live pitches and solid mid-rotation upside.

With apologies,

Yours truly,

Jason Wojciechowski