FanPost

Inside Jesus Aguilar's second half

It was a breakout 2018 season for Brewers first baseman Jesus Aguilar. Finally getting his first shot to play every day in the Majors, Aguilar demolished 35 home runs and finished the season with a .890 OPS and 134 wRC+ in 566 plate appearances. Aguilar was arguably an elite hitter during the first half of the season. 24 of his long balls came in the first half, and he posted an eye-popping 160 wRC+ and .995 OPS. Those numbers got him to his first career All-Star Game.

In the second half, however, Aguilar’s production nosedived. He hit just 11 home runs and hit for a pedestrian .760 OPS. He did still put up a 101 wRC+ after the All-Star break, so it’s not like he became useless. However, it was a far cry from his monster first half. Let’s dig into the numbers to find out why Aguilar’s season progressed as it did.

First, let's establish something. Aguilar was on an unsustainable pace in the first half. 27% of the fly balls he hit were turning into home runs, which wasn't going to continue. Aguilar was never going to hit nearly 50 home runs in a season. However, his second-half struggles can be chalked up to more than just regression to the mean.

Let’s begin by checking if anything about Aguilar’s approached changed. Strikeouts were not the problem for Aguilar in the second half; in fact, he actually struck out less. After whiffing 26.3% of the time in the first half (above league-average, but not bad), he was set down on strikes 24% of the time for the rest of the season. His walk rate essentially remained the same; he walked 10.4% of the time in the first half and 10.0% of the time in the second half.

Did he stop hitting the ball hard? Not really. After making hard contact 45.4% of the time in the first half, Aguilar made hard contact at a 42.3% rate in the second half. Those are both well above-average marks.

However, there's a little more to the story. While he did continue to hit the ball hard, Aguilar wasn't launching baseballs like he was in the first half. His fly ball rate plummetted from 45.4% (first half) to 35.6% (second half). Those fly balls, many of which became home runs, turned into popups and ground balls. Aguilar's infield fly ball rate jumped from 3.4% to 10.3%, and his groundball rate spiked from 30.6% to a second-half rate of 41.1%. This would explain Aguilar's second-half power outage.

The increase in ground balls would also explain the drop in Aguilar's BABIP. His second-half BABIP of .283 is below the typical average of .300, but it doesn't really indicate bad luck in Aguilar's case. It's no secret that Aguilar is not fast, so he's not going to leg out any infield singles. A low BABIP makes sense if he's hitting more balls on the ground.

Did Aguilar change his approach? Not really. However, something caused him to stop hitting as many fly balls as he did in the first half. That something was an adjustment made by the pitchers he was facing. Let's take a look at how pitchers handled Aguilar this season. Aguilar did his best work this season against fastballs, slugging .610 and hitting 20 of his home runs. Here's a heat map showing how pitchers attacked Aguilar in the first half with their fastballs.

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Aguilar's reputation coming into the year was a strikeout-prone slugger who sometimes struggled against fastballs. In 2017, Aguilar was merely "okay" against fastballs. He slugged .478 with a 26.1% strikeout percentage. It made some sense to attack him with velocity down the middle. However, Aguilar turned into a beast against fastballs in 2018. In addition to the previously-mentioned 20 home runs, Aguilar had a whopping .428 wOBA against fastballs while drastically reducing his strikeout rate against them to 16.7%.

Pitchers took notice of this improvement. Instead of being aggressive and attacking Aguilar with heat down the middle, they made an adjustment. Here's a heat map of the fastballs Aguilar saw in the second half.

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Pitchers began throwing more fastballs down in the zone to Aguilar, and they stopped throwing them right down the middle. Part of Aguilar's success in the first half came from pulling the baseball. 48% of his contact in the first half was pulled, and 18 of his 35 home runs (51%) were hit to left field. In case you didn't know, Aguilar can absolutely murder baseballs by pulling them.

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Naturally, the adjustment for opposing pitchers was to stop giving Aguilar pitches he could pull. They began to make this adjustment around July, and it initially hit Aguilar pretty hard. He continued trying to pull the ball (44.3% pulled contact in July) and began hitting more ground balls. He made hard contact just 32.2% of the time in July and hit line drives just 19% of the time. Aguilar was too pull-happy, and pitchers had taken advantage.

Then, in August, Aguilar made an adjustment. He began pulling the ball significantly less. In fact, he hit the ball up the middle just as often as he went to left field (both 35%). He also began going to the opposite field more often (29.4%). He went back to crushing the baseball, making hard contact 52.9% of the time. His 131 wRC+ in August was in line with his full-season mark.

Unfortunately, Aguilar seemed to run out of gas in September, when he hit an overwhelming amount of ground balls. Grounders represented a whopping 50% of his contact. He seemingly forgot how to go to right field, going opposite field only 17.6% of the time. It's possible that the poor September can be chalked up to fatigue, as 2018 represented the longest season of his career.

Pitchers made an adjustment to Aguilar about midway through the season, and he did show some signs of adjustments. Ultimately, he needs to prevent his pull-happy tendencies from getting the best of him. Aguilar showed the ability to go to right field at times throughout the season. How willing he is to do it could be a big factor in how he produces next season. To put it simply, he needs to do more of this:

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Or, better yet, this:

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Aguilar has a smooth swing, and he's strong enough to hit baseballs hard without a max-effort hack. His pull-heavy approach worked well when pitchers were giving him pitches to pull. However, as his opponents have adjusted their location against him, Aguilar could benefit from utilizing an opposite-field approach more often. He's got the power to crush the baseball to all fields; it's just a matter of him being willing to go with the pitch. Using the opposite field more frequently could be the first step to solving his second-half struggles and legitimizing his breakout season.

Statistics are taken from FanGraphs and Baseball Savant.