While there are other Braves regulars (and former regulars) who are faring worse at the plate than Jason Heyward this year, his lack of output on offense has received a great deal of attention. For a player as talented as Heyward, we expect much more than a wRC+ of 85 and an ISO of .096. So what is driving his lackluster offensive output?
A useful starting place is to examine his peripheral stats, namely BB% and K%. If he were walking a lot less than he usually does or striking out a lot more, that would be an immediate red flag that something may have changed in his approach that is negatively impacting his performance. However, that does not appear to be the case. This season he is walking in 11.1% and striking out in 19.5% of his plate appearances, compared to career rates of 11.4% and 20.5%, respectively.
Since his walk and strikeout rates don't seem to be major culprits, the natural next step would be to see what happens when he puts the ball in play. Let's look at his 2014 batted ball results separately for grounders, liners and fly balls as compared to his career average and the 2014 MLB average. Each line of statistics (current through the end of Tuesday 5/20/2014, courtesy of Fangraphs and http://www.baseballheatmaps.com/graph/distanceleader.php)* will post his numbers in that order, separated by slashes.
Batted Ball %: 41.9% / 48.5% / 45.6%
BABIP: .296 / .250 /.235
ISO: .037 / .024 / .020
wRC+: 73 / 39 / 30
Thus, he is doing well on grounders relative to his career norms and league average. How has he fared on line drives?
Batted Ball %: 18.6% / 18.0% / 20.2%
BABIP: .708 / .737 / .691
ISO: .167 / .278 / .205
wRC+: 360 / 401 / 359
He is doing worse on liners relative to his career norms, but is right around league average with a higher batting average and less power.
On balance, Heyward has been above league average and roughly in line with his career averages on grounders and liners combined. As you might expect, then, he has fared very poorly on fly balls.
Batted Ball %: 39.5% / 33.5% / 34.2%
BABIP: .042 / .112 / .131
Avg: .100 / .239 / .221
ISO: .200 / .512 / .403
wRC+: -3 / 158 / 121
IFFB%: 11.8% / 12.9% / 9.7%
HR/FB: 5.9% / 14.4% / 10.1%
Avg FB Distance: 276.82 / 290.99 / 279.11**
Relative to his career, Heyward has basically traded ground balls for fly balls this season. His fly ball rate is the highest of his career and is above the 2014 league average. Given his impressive power (career 14.4% HR/FB), we want him to be hitting his fair share of fly balls. However, it is starkly clear from the numbers that that has not worked out well for him so far this season. Why?
First of all, the increased number of fly balls have been leaving the park at roughly 60% of the league average rate and 40% of Heyward's career average rate. His average fly ball distance is noticeably below his career average and is below the 2014 MLB median, so it may be reasonable to expect some drop off in HR/FB. However, as his average FB distance has trended downward every year (a topic for another post), his HR/FB has never dropped below 13.0% for a full season, going as high as 16.9% in 2012 (his 27 HR year). Thus, it seems unlikely that his HR/FB would be expected to fall so sharply despite the decline in average distance.
Since Heyward is hitting more fly balls and less of them are leaving the park, outfielders have had relatively more chances to field them. And boy have they! His BABIP on fly balls is 3/8 of his career average and 32% of the 2014 MLB average. Given these huge disparities, it is necessary to check his rate of infield flies. While he does have a higher IFFB% than the league average (which is not unusual for him historically), it is not alarmingly high and is in fact below his career average. Thus, the massively low BABIP does not seem to be driven by a higher than usual rate of popups.
What effect is this low fly ball BABIP having on his offensive numbers? His fly ball batting average and isolated power (ISO) are less than half of his career averages or the 2014 league averages, giving him a fly ball wRC+ of -3 (that's right, negative 3!) relative to career and league averages of 158 and 121, respectively. An increase in fly ball rate paired with such a precipitous drop in fly ball wRC+ has severely harmed his offensive production, despite his being above average on ground balls and line drives combined. It has especially hurt his power numbers since fly balls that land safely almost always go for extra base hits.
So what should we expect going forward? Given that Heyward largely appears to be the same hitter he has been, his luck on fly balls should improve significantly (both in fly balls leaving the yard, and finding grass when they don't). Thus, even if his luck on grounders worsens somewhat, his batting line should rebound markedly. Sometimes players run into prolonged stretches of rotten luck even when their true talent level hasn't really changed, and I believe that is a large part of the story with Heyward this season. There is no particular reason for concern - he should be just fine!
* Big thanks to fellow TC commenter Ivan the Great for the distance leader data, and for taking the time to present the stats in this post in a much more easily readable form at http://imgur.com/8te5JTj.
** The 2014 league value is the median instead of the average, since it is much more convenient to calculate given the online data presentation. This shouldn't change the picture significantly.