(All stats are current as of the end of April 24th.)

During the offseason, Jason Heyward and Troy Tulowitzki were two of the highest-profile players on the trade block. Heyward was ultimately dealt as the Braves gear up for the future and the Cardinals look to fortify RF after the passing of Oscar Taveras. Tulowitzki was not dealt, as the Rockies hope that they can make an improbable run to the playoffs. Both players could be looking for new homes within the next year, as Heyward hits free agency (barring an extension) and Tulowitzki would be a very tempting target at the trade deadline or in free agency.

However, both players have started the season slowly. While Tulowitzki has a 103 wRC+ (which is pretty darn good for a SS), that figure is far below his 2014 results (171 wRC+) and his career figure (125 wRC+). Much of the blame can be placed on his .197 ISO, which is far below both his 2014 and career ISO. Tulowitzki has been able to counteract the drop in power somewhat due to a .370 BABIP that is far above any BABIP he has recorded over a full season. Heyward’s drop has been even more severe, as he is the owner of a B.J. Upton-esque 64 wRC+. While much of that should be attributed to a paltry .235 BABIP, some blame also can be ascribed to a poor batted ball distribution. However, it is too early to say that either player won’t see these trends reverse as the season progresses.

On the other hand, both players are suffering a precipitous and concerning decline in their plate discipline. Tulowitzki’s K rate has shot up from between 15 and 16 percent to almost 24 percent. Likewise, his walk rate has fallen to a paltry 1.6 percent as he has drawn one walk over the season. That shift is being driven by an increase in his swings on pitches out of zone, which has grown to 35 percent from 27 percent in 2014 according to Pitch F/X data:

In addition, Tulowitzki is making less contact as he swings, as his contact rate is below 80 percent – a percentage he has never had at the end of the season. He is also swinging and missing more and is over the league average for the first time since his disastrous cup of coffee in 2006. Tulowitzki’s also seen 8 percent more pitches in the zone (a higher figure than ever before), which indicates that pitchers are not as afraid of him as they once were. All of this comes directly after he had hip surgery, which suggests that he may not be fully recovered yet or that the injury may have eroded his skills slightly.

Heyward also has seen his plate discipline deteriorate but not to the same level that Tulowitzki has. First the good news: his strikeout rate, while slightly elevated from his totals in the past few years, is still in line with his career norms. However, the rest of his plate discipline numbers are worse than his career numbers. As noted by Bernie Miklasz, Heyward only has one walk, is swinging at far more pitches out of zone than ever before, and is seeing fewer pitches in the zone than ever before. Miklasz also notes that Heyward is pounding groundballs – he is currently putting 62 percent of his balls in play on the ground. This is far above his career averages (as shown in the chart below) and is a sign that chasing more pitches is not helping him generate power.

In addition to the points that Miklasz made, Heyward is also swinging far less at pitches in the zone. This season, he has swung at 58 percent of pitches in the zone, the lowest percentage since his rookie year. These diverging trends have allowed Heyward to set a personal record: for every pitch that Heyward swings at out of the strike zone, he only swings at 1.04 pitches in the strike zone.* This is far below his career ratio of 1.69.

Now, as loyal FanGraphs members (only the truly committed read the Community board!), I can hear your refrain of “Small Sample Size.” And I certainly agree that it is too early to completely believe in the magnitude of these changes. It is extremely unlikely that both players will walk less than 2 percent of the time this year. However, I believe that the magnitude and consistency of the changes is a clear sign that both players are suffering due to the erosion of their plate-discipline skills. Both players have reached the stabilization point for strikeout rate, are halfway to the stabilization point for walk rate, and Heyward is quickly approaching the stabilization point for groundball rate. In addition, per pitch metrics like O-Swing and Z-Swing stabilize quickly, with swing rate stabilizing at 50 PAs. While those stabilization points only denote the point at which the data is half noise and half signal, the changes are consistent enough across multiple measures of plate discipline that its extremely hard to argue that it could **all** be a fluke. While both of these players are plus defenders and have the power to still be plus hitters with poor plate discipline, their value will suffer unless they can find a way to turn around their plate discipline.

* This statistic can be calculated using the following formula: (Zone%*Z-Swing%)/((1-Zone%)*O-Swing%).