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Astros Offense vs. Lefthanded Pitching

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Red Sox May Use 4 LH Starters

MLB: Houston Astros at Boston Red Sox Bob DeChiara-USA TODAY Sports

The MLB Network interviewed a beat reporter from Boston yesterday. He said that the Red Sox believed that they might get an advantage over the Astros by starting four left-handed pitchers in the ALDS. Apparently, the basis for this strategy is the Astros’ less successful record against left-handed starting pitchers. The Red Sox not only would start lefties Sale and Pomeranz in games 1 and 2, but also might start lefthanders in games 3 and 4.

Should we be panicking because the Red Sox will throw so many lefthanded pitchers at the Astros? Short answer: No.

It’s unclear what facts are relied upon to conclude that the Astros enjoy less success against left-handed pitchers. However, I will ignore any argument based on win-loss decisions in games started by lefthanders—for much the same reason that win-loss record is a poor indicator of pitcher run prevention performance. If you are a TCB reader, I suspect you already know this. But, just to illustrate, if certain opposing pitchers have a good W/L record against the Astros, it could be due to sub-par performance by the Astros’ pitcher(s) in those games. And the performance of Astros’ pitchers has nothing to do with how well the Astros’ hitters performed against the opposing pitcher.

While it is true that the Astros’ offense is slightly better against RHP, the difference is minimal. The advanced stats against. RHP are here, and versus LHP, here. The wRC+ against LHP is 120, and 122 against RHP. The ISO and SLG is slightly higher against RHP, and OBP is slightly higher against LHP. The LHP OPS is .814, and the RHP OPS is .827. The peripheral stats (K%, BB%) are quite similar. Moreover, the Astros rank No. 1 in the major leagues against RHP, and they are tied for first (with the Tigers) against LHP. This is hardly supported for any contention that the Astros’ offense is weak against LHP.

However, if the measurement is based on runs scored against LHP, we find somewhat greater splits. I calculated runs per plate appearance for each team versus LHP and RHP and then multiplied the ratio times 40 plate appearances to estimate runs scored per game. The Astros’ runs per game against LHP is 5.29, and 5.85 against RHP. The approximate one-half run differential is not insignificant and exceeds the difference we might expect based simply on the wRC+ differential. Since the Astros apparently were less efficient against LHP in converting offensive actions into runs, this type of comparison may support the contention that LH pitchers hold an advantage over the Astros.

But, countering this point, the Astros’ splits for runs per game does not indicate that the Astros are weak against LH pitching. In fact, the Astros’ runs per game is third best (behind the Tigers and Rockies) against LHP.

Why were the Astros relatively less efficient at scoring runs against LHP? The usual culprit is the timing of offensive actions. The Astros’ offense was much better versus RHP in high leverage situations. (Link and Link) The wRC+ was 154 in high leverage situations vs. RHP, and 142 vs. LHP. The Astros’ offense was very good in high leverage situations against LHP, and it was spectacular against RHP in those situations. The Astros were more clutch against RHP.

Does this distribution of clutch performance between RHP and LHP situations indicate that the Red Sox strategy has identified an Astros weakness? I think not.

  • The Astros’ offense performed at a high level in clutch situations versus LHP. The Astros’ wRC+ ranked No. 1 in high leverage situations against LHP.
  • Clutch hitting splits are very susceptible to regression. The magnitude of the differential between the LHP and RHP high leverage split is likely to regress over time.

The Astros (and their fans) shouldn’t be frightened by an LHP strategy. That’s not to say that they shouldn’t be concerned about facing Sale and Pomeranz; however, the concern should be based on the high caliber of those two pitchers, rather than just their left-handedness. Moreover, if the Red Sox were to eschew higher quality pitchers in order to put four LHPs in the ALDS rotation, that strategy could backfire.