What Has Happened to Jesus Aguilar?


Note: All statistics are accurate as of April 23, 2019 and courtesy of FanGraphs.

At the 2018 All-Star break, Jesus Aguilar appeared to be a breakout star. He had at times single-handedly carried the Brewers’ offense with a first half slash line of .298/.373/.621 while hitting 24 home runs and 70 RBI. He effectively elevated his profile from back-up first baseman to potential MVP candidate. Just three months removed from being arguably the Brewers’ third option at first base, his performance had landed him a spot in both the Home Run Derby and the All-Star Game.

However, Aguilar turned into a shell of this breakout star in the second half of the year. His performance dipped dramatically across the board, with his slash line post-All Star Game being .245/.324/.436. The dip continued into 2019, as he is currently hitting .132/.231/.162.

Weighted-runs-created-plus (wRC+), an advanced statistic used to measure a player’s offensive performance, displays Aguilar’s 2018 second-half drop-off in better detail. While he posted a wRC+ of 160 prior to the All Star Break, his wRC+ post-All Star Break was only 101. This means that he was 60% better than the average player offensively in the first half, and only 1% better in the second half.

This isn’t news for Brewers’ fans. Everyone knows Aguilar has been struggling. That change in wRC+ only tells us what has happened, not why it has happened, which is the question fans want answered. So, on a deeper level, what has changed for Aguilar since last year’s monster first half?

To analyze this question, I will compare Aguilar’s first half rate stats from last year to everything since. This combines his performance thus far in 2019 with his second-half 2018 performance. Just a heads up: things are about to get “mathy”.

First and foremost, on a basic level, he has seen large decreases in his pull and fly ball rates. His pull rate has decreased by 10%, with his fly ball rate seeing a decrease of 9%. Also, his pull rate of fly balls specifically has decreased by 17%. That he is pulling fewer of his already decreased number of fly balls compounds this issue. This is a significant problem, as Aguilar hits a majority of his home runs to left field. This simultaneous decrease in fly balls, pulled balls, and pulled fly balls is likely a driving factor in his decreased home runs totals and cumulative offensive output.

In addition, Aguilar has seen a 13% decrease in his home run per fly ball rate (HR/FB) since the first half of 2018. While he posted a HR/FB rate of 27% prior to the All-Star break, that number has dipped to 14% since. This means that while he previously hit a home run with about one in every four fly balls, that is now more like one in every seven. Given that his fly ball rate already decreased by 9% as discussed above, the effect of that decrease on his offensive output becomes multiplied by his diminished HR/FB rate. In short, a much lower number of his already decreased amount of fly balls are turning into home runs.

In place of his fly balls, Aguilar is hitting a significantly higher number of ground balls. His ground ball rate has increased from 31% in last year’s first half to 40% since. Ground balls are easily the least valuable batted ball on average, with an average OPS league-wide of .494 compared to line drives (1.574) and fly balls (.915). Clearly, a shift in Aguilar’s batted ball portfolio from fly balls to ground balls would drive down his offensive output. Increasing their occurrence has done him no favors.

To summarize, so far we have established the following:

  • Aguilar has been hitting far fewer fly balls and pulling fewer fly balls, leading to fewer chances for home runs.
  • When Aguilar has been hitting fly balls, they are turning into home runs far less often.
  • In place of fly balls, Aguilar has been hitting more ground balls, which provide the least value of any batted ball by a wide margin.

Aguilar’s ineffectiveness can be explored further by looking at his batted ball heat maps. These will show how he typically performs in each area of the zone, and which areas (if any) are giving him more trouble than they had previously. It is worth noting that he has faced the exact same number of pitches over the two time frames (Pre-2018 All-Star Break, Post-2018 All-Star Break), so these numbers should be very representative of the changes to his batted ball outcomes. Below are Aguilar’s slugging percentage per ball in play heat map divided into a 5-by-5 grid (courtesy of FanGraphs). On the left is his map from the first half of 2018, and on the right is everything since.

As you can see, the hot areas Aguilar used to have in the middle of the zone have turned cold. The fall in the “middle-middle” zone has been most drastic – his slugging percentage on balls in play there has dropped from 1.536 to .406. Roughly translated, prior to the 2018 All-Star Break, he would most often get an extra base hit when he put a ball in play from that zone. Since then, the typically ball in play ends with him not getting on base.

It is easy to see from the map that he is struggling across the board and not in just one area of the zone. While he used to have 7 hot zones within the strike zone, since last year’s All-Star break he has only had 2. This means that his change in production is not limited to just one specific location that pitchers are zeroing in on.

Finally, is there any one pitch that has given Aguilar more trouble than it had previously?

While Aguilar’s results against each pitch type have dropped across the board, there is one type of pitch that stands out from the rest: sliders. In the first half of 2018, Aguilar slugged .651 against sliders with 7 home runs. Since then, he has slugged a meager .311 with 4 home runs. His batting average on balls in play facing the pitch has decreased from .333 to .143, indicating either he is facing extremely tough luck or he is simply not generating good contact against the pitch. Due to the trends we have discussed above, I would guess that it is most due to a lack of good contact.

So, where does Aguilar go from here?

There is no one right answer to this question, and honestly, I am grossly under-qualified to answer it. However, it is clear that Aguilar had performed better when his ground ball rate was lower and he was hitting more fly balls. The solution to correcting that is not easy – there is a lot from a hitter’s perspective that goes into generating high quality contact at a desired launch angle. However, if there is an area for Aguilar to focus on, it would be to use whatever methods are available to move back towards the batted ball profile that brought him so much success in the first half of 2018. This may be mechanical-based, approach-based, or simply pure luck, but for Aguilar to be an effective Brewer in 2019, it appears that changes need to be made.


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