Diamond's thin margin of error

Scott Diamond in the dugout after being pulled in the
fifth inning on Sunday afternoon in Toronto.

Scott Diamond had another poor start Sunday: 4.2 innings, six runs, three homers, four walks.

Last season's rotation revelation has seen his ERA rise almost two full runs per game — 5.52 this year, 3.54 in 2012 — and he has now failed to get through six innings in five of his last six starts.

It's not easy for me to be objective about Diamond, who I declared earlier this year to be MFT — "My Favorite Twin." I have a basic liking for this ilk of pitcher: a lefty who avoids walks and homers and lives on the groundball. I've had that fondness from the days of Geoff Zahn, who averaged more than 200 innings over a four-year period with the Twins in the late '70s and went 53-53 in that span — and did so with a strikeout rate lower even than Diamond's.

Such pitchers are very team-dependent. They need infielders who can turn the double play, catchers who can help hold the running game in check (to keep the double play in order), outfielders who can cut off balls in the gap (to keep the double play in order).

Diamond's season to date illustrates how thin the margin of success is for a low-strikeout pitcher.

Diamond is walking 2.1 men per nine innings this year; a good rate, but a half-walk higher than in 2012. His strikeout rate is down from 4.7 K/9 to 4.3. His BABIP — batting average, balls in play (which takes out the home runs and the strikeouts) has risen 36 points, which suggests that year's defense behind him has been less effective. The home run rate has boomed from 0.9 HR/9 to 1.5, which is too high for any type of pitcher.

And, perhaps most telling, Diamond ground-ball to air-ball (flies and liners) ratio has fallen from 1.17 to 0.90. That's still above average (0.82), but not much above. (All those numbers are gleaned from Baseball Reference this morning.)

Diamond last year was among the very best in the majors at getting ground balls, at getting double-plays, at avoiding walks. The things that pitchers with his skill set have to be good at, he was generally very good at. This year, for whatever reason, he isn't. And for him, that's the difference between success and failure.