Consider a hitter who has the following hot/cold zone profile by RAA/100, a representation of how successful they are when putting balls into play in a particular pitch location:

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This is the zone of a pure pull hitter. Per Fangraphs, Didi has a wRC+ of 204, 231, 229 to his pull side last three years. Hitting oppo, as noted baseball commentators like Suzyn persistently urge Didi to do, he is putting up a wRC+ of 71, 27, 76. That’s going from Barry Bonds to a pitcher.

A more luck neutral measure like xwOBA has him at 0.291 oppo side and 0.384 pullside last 3 years.

This type of oppo/pull split is not unusual. Some of the most productive hitters in the game have huge oppo/pull splits. Pitchers invariably target the outside of the zone vs these players just to avoid their productive hitting zones. Against a player like Mookie Betts or Didi, if a pitch is put into play from the cold zone, it is mission accomplished for the pitcher.

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Well uh, Didi has been attacked as a pure pull hitter and he’s responding by swinging at everything thrown at him. Pitch% zone on the left, swing% zone on the right, and they look the same. This is in pretty stark contrast to his swing% zone from his otherworldly April:

April 2018 Pitch%(L), Swing%(R) for Didi Gregorius

While he was still swinging at outside pitches in April, pitchers were not exclusively pitching him on the outside part of the plate. He was able to hit a fair number of balls in the middle to inside part of the plate, where he could do damage. This change in zone profile is backed up by a stark shift in batted ball spray(air balls only):

Spraychart Before May 1st (L), Spraychart After May 1st (R)

As mentioned before, Didi is elite when he pulls the ball in the air, but he does not hit the ball hard enough to be productive going oppo. In April, he had a 0.607 xwOBA on all line drives and flyballs, after May 1st, 0.357. While Didi has had a strong flyball profile for his career, he particularly relies on those flyballs happening to his pull side. The reason for this is pretty simple, he doesn’t hit the ball hard enough anywhere except to his pull side. His splits to center, per fangraphs classification, is at a similarly abysmal wRC+ of 61 for his career.

This is how you get drastically different production with the same hard hit rate and so on, batted ball direction matters for a lot of hitters. Didi is actually hitting the ball fairly hard after April, but those same balls are going to the left field expanse instead of the comfy right field corner.

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After spending a torrid April not chasing out of zone pitches, particularly outside pitches, Didi has reverted to see ball hit ball, even when the ball is in the other batter’s box. If you are Didi and you want to be good at baseball, consider framing the above chart and hanging it on your wall.

For a pure pull hitter like Didi, plate discipline is even more critical, as he needs to not only limit himself to swinging at strikes, but ideally waiting for a pitch in his power zone. Putting a ball on the outside edge into play is not enough when your entire production is on the pull side. Pitchers will continue to attack Didi on the outside part of the plate as long as he’s putting those balls into play, or worse, chasing them out of the zone. The harm is not just how badly he does on those pitches, but letting pitchers work a strategy that severely limits the amount of pitches he can actually do damage with.

One does not go 0 for 40 without bad luck, but Didi’s slump revealed some real problems about his game. Pitchers are and will continue to attack his weakness on the outside part of the plate, and if he can’t adjust, it does not bode well for the future. Why throw him a ball he can pull when you can just get him to hit 300 feet flyballs to the left fielder?