Yoenis Cespedes burst onto the scene in 2012 with an incredible rookie campaign, and if not for Mike Trout, would have captured the Rookie of the Year award. He posted a wRC+ of 136, an ISO of .214, and a wOBA of .368. With home runs like this, it looked like Cespedes was destined for greatness, but his next two seasons were markedly less impressive. His wRC+ values in 2013 and 2014 were 102 and 109 respectively; his ISO .202 and .190; and his WOBA .318 and .326

Playing for Boston in his final 51 games last season only steepened his downward spiral. In 213 PA’s, Cespedes only hit five home runs, an ISO of .159, and an OPS+ of 100, exactly league average. All of these metrics are cause to doubt that Cespedes will be able to live up to his new team’s expectations heading into 2015.

Cespedes’ decline is a textbook lesson in how complex hitting is. While nothing drastic has changed in his approach, the many minor issues have lead to big issues. As examined below, a chain of small changes, from the frequency of the pitches he swings at, to the consequence of where he makes contact, have conspired to make a former slugger into nothing more than a league average hitter.

While Cespedes has never been one to walk a lot, his last two seasons have yielded noticeable drop-offs in his BB%. In 2012 he walked 8% of the time, which according to Fangraphs is exactly MLB average. In 2014, that figure fell to 5.4% for a change of 32.5% overall. Cespedes’ walk rate was acceptable to start his career, but has now sunk to unacceptable levels just three years into his MLB tenure.

His declining BB% rates may have something to do with his increasing O-Swing%, which has risen for two consecutive years, albeit not as drastically as his walk rates have fallen.

His O-Swing% has always been above the MLB average of 30%, but as Fangraphs points out, there really isn’t an ideal plate discipline configuration that predicts success. Having said that, Cespedes’ O-Swing rate has increased each year. He has been inviting pitchers to attack him by throwing more and more pitches off the plate with the hope that he’ll chase.

More of a problem than his O-Swing% however is his O-Contact%. In 2012 his percentage was 59.5%, but in 2014 that number rose to 67.10%, an increase of 12.77%. Cespedes is dealing with two variables that are compounding to bad effects. He has begun to swing at more pitches off the plate, while simultaneously making more contact with them.

As his contact% has changed, so too have his batted ball ratios.

Cespedes’ LD%, GB%, and HR/FB ratio have all fallen, hurting his overall offensive value. His declining LD% can help explain his plummeting ISO, while his GB% has affected his BABIP values. The most notable drop-off has undoubtedly been Cespedes’ HR/FB ratio, which in 2012 was an above average 14.80%, but in 2014 was just a tick above average at 9.6%. The curious part of this result is that since 2012, while his FB% has increased each year, his ISO has dropped consecutively for two seasons by a total of .024. This doesn’t seem to make sense because higher FB% usually correlate to higher ISO values, but as I discuss below, Cespedes has other factors at play.

Those other factors are the exit velocity and the elevation of balls off his bat.

Unfortunately for Cespedes, all of these individual measures are trending in the wrong direction. The average exit velocity of his home runs, which in 2012 was 106.41 MPH, fell by almost 5 MPH by last season to 101.71 MPH. Simultaneously, balls are leaving his bat at higher angles of elevation by 8.53%. Decreasing exit velocity and increasing elevation have caused the trajectory of Cespedes’ fly balls to change. They climb more steeply, but for less distance overall.

Consequently, the average "true distance" of his home runs, which ESPN defines as if the ball "flew uninterrupted all the way back to field level", has decreased from 409.08 feet in 2012, to 387.54 feet in 2014; a total loss of 21.54 feet, and a decline of 5.3% Baseball Heat Maps agrees with this finding not just for the average distance of his home runs, but also for his fly ball and line drive distance in general. Excluding home runs, Cespedes’ fly balls and line drives have lost an average of 7.57 feet since 2012.

Earlier I mentioned the oddity of his increasing FB% coupled with a decreasing ISO, which generally doesn’t make sense. However, Cespedes is again dealing with two factors that compound to a negative effect. He’s hitting more fly balls, which should improve his ISO, but because of the change in trajectory, his fly balls are actually leading to a lower ISO.

Adding to Cespedes’ issues, is the fact that he’s seemingly no longer able to do damage against the fastball.

In 2014, Cespedes’ produced a negative pitch value against the fastball for the first time in his career, and an overall 10.2-point drop. He made great strides against the slider and curveball, but when a hitter in his age 28 season can no longer hit a pitch he sees 53.7% of the time, it’s cause for alarm.

The only apparent positive it seems for Cespedes heading forward is that he’s no longer going to call O.Co or Fenway Park home. In 2014, both of those environments suppressed home runs. Oakland had a HR Park Factor of .903, while Boston was even worse at .720. In contrast, Comerica Park had an above average HR factor of 1.014. Going into a contract year, it’s imperative for Cespedes to reverse the trends of the past two seasons in order to secure another payday. Hitting is incredibly complex, and as shown, there are a multitude of factors at play. It remains to be seen whether or not he’ll be able to correct these issues, but nonetheless, it will be fascinating to keep an eye on him in 2015.

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All statistics courtesy of FanGraphs, ESPN Hit Tracker, Baseball Heat Maps and Baseball-Reference.

Matt Goldman is a Featured Writer for Beyond the Box Score. You can follow him on Twitter at @TheOriginalBull.