Manny Machado and Starlin Castro used to be extremely comparable

It may be tough to fathom now, but AL East infielders Manny Machado and Starlin Castro have had a lot in common. Since 2015, Machado has accumulated just over 10 fWAR, which would suggest that he has literally been ten times as valuable as Castro, who has just one WAR in that timeframe.

At one point, there were several similarities between Machado and Castro. Manny Machado was the third overall pick in the 2010 MLB Draft, behind Bryce Harper. Starlin Castro also garnered a lot of prospect hype, as both of them were in the big leagues shortly after their 20^th birthdays. Both players were looked at as cornerstones in the future of their respective franchises.

Offensively, the two infielders would occasionally fall into the same bad habits. Both Machado and Castro struggled with plate discipline early in their careers, and they also struggled to get the ball in the air, taking away from the exciting amounts of pop in their bats. After dealing with their fair share of ups and downs in previous seasons, they both appeared to be on the verge of something special after the 2014 season. Here are a few telling stats from that year:

That is where the similarities really began to disappear. Machado had undergone season ending knee surgeries two years in a row, but was still considered a potential superstar if he could stay healthy. Castro's name was brought up in trade discussions with the New York Mets, with Castro for Zack Wheeler rumors flying across countless Twitter feeds. Unfortunately, the two infielders had considerably different seasons in 2015:

Despite hitting fewer line drives than he did in 2014, Machado improved his plate discipline considerably and managed to hit more flyballs. After hitting 33 home runs from 2012-2014, he hit 35 in 2015. On the other hand, Castro fell back into his old habits, hitting way too many grounders and going fishing on pitches out of the strike zone.

This year, Castro has has found his power stroke early on in the season. With seven home runs and an ISO of .170, he is on pace to set career highs in both categories. At times, he has been the only sparkplug for the listless Yankees offense. Unfortunately, his on base percentage is under .300 for the second straight season. Because of his failure to get on base and the occasional base running error, Castro's offensive production has been below replacement level in 2016.

However, Machado's MVP-caliber season provides hope for players like Castro. It is true that Machado has a much, much higher ceiling. The mere fact that Machado is presumably bigger than he was in high school, has had two knee surgeries, and has gone back to playing shortstop like it's nothing shows just how good of an athlete he is. But Machado is showing that better plate discipline and fewer grounders can go a very long way.

Of course, hoping hitters suddenly develop plate discipline is a very frustrating ordeal. Even highly accomplished players like Yoenis Cespedes, Josh Hamilton, and Pablo Sandoval have drove their respective fanbases to the brink of insanity by swinging at pitches outside of the strike zone. But in the case of players like Manny Machado, his newfound flyball and line drive approach doesn't seem to be linked to his improved plate discipline:

Looking at data from the last five seasons, there is a positive, but very weak correlation between swinging at pitches outside of the strike zone and hitting excessive grounders. It would follow that Machado probably made some mechanical adjustments to hit the ball in the air more often, in addition to improving his plate discipline. Even if someone like Starlin Castro can't learn to lay off bad pitches, there is still hope for him to hit more line drives and flyballs by altering his swing plane.

Early in their careers, Manny Machado and Starlin Castro were promising infield prospects who had bright futures ahead of them. While Machado looks like he will sign a $300 million contract in a few years, Castro has stumbled a bit since the early part of the decade. But if he can make the same adjustments Machado did, he could definitely live up to the hype he generated in Chicago.

Data is courtesy of Fangraphs.