Contact quality concerns are coming true for Aledmys Diaz

In the off-season, I tagged Aledmys Diaz as someone who could benefit from a conscious effort of hitting more fly balls. He posted a well-above average average Exit Velocity in 2016, but his average Exit Velocity on fly balls and line drives was merely average. If he could change the plane of his swing so that hard contact more often meant a fly ball, he could sustainably hit for more power.

At the same time, there was little risk for Diaz to pursue a more power-friendly approach. While Diaz posted a .312 BABIP in 2016, my xBABIP calculation gave him just a .266. That’s thanks to the fact that he hit an above-average amount of pop-ups as well as low-angled grounders. A more fly-ball heavy approach would probably create more pop-ups, but also less low-angled grounders, mitigating the cost a little bit.

Diaz’s ground-ball percentage (GB%) has held steady this year, just above 45%, which is a little more than average. To his credit, he has increased the difference in average Exit Velocity between flies and liners and grounders, from 4.2 MPH to 6.1 MPH. However, his overall average has dropped, rendering that gain moot. Diaz’s 85.3 MPH average is almost 5 MPH worse than last year’s 90.2 MPH.

Diaz doesn’t only have a problem in terms of quality of contact, but also in terms of plate discipline. As The Red Baron mentioned a couple of weeks ago, his O-Swing% (the rate at which pitchers swing at pitches out of the zone) has sky-rocketed. However, while it’s still of concern, he’s starting to settle in a bit. Here’s a rolling 10-day average of both his O-Swing% and Z-Swing%, provided by the good people at Fangraphs:

As the graph entered 2017, both rates shot-up to career highs. At least temporarily, Diaz brought his O-Swing% back down to his career average (though it should be noted, the huge spike brought his O-Swing% average up as well). Increases in O-Swing% were always accompanied by similar increases in Z-Swing%, and his contact rate has remained the same from 2016 to 2017. Diaz should probably be seen as a bit more of a free-swinger than before the year, but April probably isn’t his new self.

The bigger issue to me is the aforementioned contact quality. I saw regression in Diaz’s contact quality, and so did one of Statcast’s new stats, xwOBA. Similar to my own xwOBA I developed last year, Statcast’s version utilizes the average outcome from a batted ball at each combination of Exit Velocity and Launch Angle. Using the average outcomes, we can find what a hitter’s production “should“ have been, based on his assortment of statcast-recorded batted balls. This neglects to utilize a player’s speed or how easy it is to shift against them, but it gets us most of the way there.

The problem with Diaz is that xwOBA didn’t like him near as much in 2016 as his results did. Of 429 players with more than 100 at-bats last year, he had the 12th biggest negative difference between his xwOBA and his wOBA . That’s inside the bottom 3%.

Diaz’s 2017 wOBA currently sits at just .295, earning him just a 79 wRC+. However, a lot that can be blamed on a month of barely ever taking a walk, something I wouldn’t expect going forward. His non-contact wOBA (his wOBA over non-contact events: walks, strikeouts, and hit by pitches) dropped from .287 to just .138 (the average is .200). It wasn’t just the contact though. His on-contact wOBA (his wOBA just in situations where the ball is put in play) sits at .312, down from .394 in 2016, and down from his 2016 xwOBA on contact of .337 (average is .370).

With Statcast calculating such similar stats themselves, I didn’t bother calculating an xwOBA on-contact for Aledmys Diaz in 2017. However, I do know that it’s lower than his actual wOBA on-contact, because his statcast-calculated overall xwOBA sits at .270, 25 points lower than his actual wOBA.

So what’s going on with Diaz? Let’s analyze his contact quality using a new feature from Baseball Savant, called Radial Charts. What’s a Radial Chart? Well, it’s easier to show you. Here’s Diaz in 2017:

This shows the six different qualities of contact according to Statcast. We’ll run through them one at a time. To read this image, think of the half-disk above as a protractor. Each spot is a specific combination of Exit Velocity and Launch Angle. Whatever the angle is from that specific spot to where the hitter made contact, is the launch angle. The farther the distance away, the more Exit Velocity. The image is split into six different regions, representing the six qualities of contact.

This might be confusing if you’ve never seen one of these before. Or rather, if you’re not confused now, then you’re ahead of where I was when I first saw one of these. So we’ll look at each category and hopefully it’ll make it clearer.

Barrels: This is the category you’re most likely to have heard of, as it’s part of Baseball Savant’s leaderboard. They’re by far the best type of batted ball. They mostly occur between 5 and 50 degrees, and with a high 90’s or better Exit Velocity. For better context, line drives are between 10 and 25 degrees, and more homers occur on line drives than you might think, with home run production peaking at 27 and 28 degrees. On average, barrels have wOBA of 1.433. These are homers, or hard to catch balls in play that are very likely to be doubles.

Solid Contact: This is the second-best category of batted balls, though they’re less than half as productive as Barrels, with an average wOBA of .692. As you notice, It only represents a tiny sliver of possible batted ball combinations, much smaller than those that qualify as Barrels. However, both represent a similar proportion of total batted balls.

Flares and burners: Flares and burners are kind of a weird category, combining hard hit balls at low angles with weakly hit balls at high angles. While neither of those things should be what the hitter “wants” to do, they do have good results, with an average wOBA of .630. For additional understanding, grounders come in at 10 degrees or lower.

Hit Under: The first below-average quality of contact, Hit Under batted balls are generally medium hit fly-balls that don’t have the high velocity to be homers or the low velocity to be considered flares. It also represents any batted ball over 50 degrees, when the angle is too high for more than a minuscule chance of a homer or a blooper. The average wOBA of this group is just .095, as it’s fairly rare that this quality of contact is a hit.

Topped: This is the opposite of hitting under the ball. Topped balls represent any batted ball under about -5 degrees. It also represents batted balls above that line, but below the velocity and/or angle requirements of being a flare or burner. The average wOBA for these is .206. That’s well-below the average on-contact wOBA of .370, but also healthily better than when the hitter gets under the ball.

Weak contact: finally, weak contact is any batted ball under 60 MPH, and does include bunts. Interestingly, it grades out better than the prior two categories, with a .460 average wOBA. The reason - I assume - is that a weakly hit ball will often involve infielders having to cover a lot of ground and make a play with little margin for error. A lazy fly ball is easier to make an out on, and a more routine grounder gets to the fielder quicker, while still needing to be perfectly placed to miss everyone.

Now that we know a little more what we’re looking at, we can take a look at what this image says about Diaz’s contact quality in 2017. Still though, unless you’ve looked at a whole bunch of these already, it might not tell you much. I haven’t either, so I figured out what the league average rate of each of these categories was, so that we can compare Diaz to league average.

In 2016, Diaz was well-below average when it came to Barrels. It may not look like a big margin, but 3.8% is 32% less than 5.6%. Considering they’re the most productive type of contact by far, that’s a big deal. Diaz declined to further below-average in 2017, but with the sample sizes in question, at this point it only means one less Barrel than his rate in 2016. His Solid Contact matches league average in both years.

He’s below-average both years in Flares and Burners, but it’s hard to fault him for that. I’m not going to be the one that says Diaz should try to hit it hard and into the ground more, or that he should hit it lightly in the air more.

In 2016, Diaz got under the ball 33% more often than the average player. That was the biggest drain on his profile. He managed to “top” the ball a less than average amount of the time. In 2017, he’s cut down on getting under the ball, but he still does so more than average. At the same time, his topped batted balls have increased to league average.

The fact that Diaz can maintain a high rate of getting under the ball along with an average rate of topping it, seems troubling. It looks to me that Diaz’s seven months in the majors indicates a lack of ability to square the ball up.

There is the fact that Diaz plays shortstop, where less offense is expected. However, on the other side of the ball, Diaz has the sort of questionable defense that is backed up by a strong bat. Without a lot of confidence in Diaz’s offense, it’s harder to excuse the defense.

There’s not much to be done about this from a roster standpoint. Diaz likely won’t be worse going forward than he’s already been in 2017, he’s just not as good as his 2016 let on. At the same time, it’s not like there’s someone waiting in the wings who could likely be better. I think Greg Garcia is under-rated a bit, but even I don’t think he should be starting everyday. However, if you’re waiting on Diaz to start producing like last year, you might want to let go of that hope.