Checking in on 2014's breakout stars

Almost two months into the 2015 season, I want to look back at the breakout stars of 2014 and re-evaluate where they stand to date. Collecting data from Fangraphs.com, I wanted to determine who were the breakout stars last season, explain what changed to cause a breakout, and see if regression has occurred to this point of the next season.

To establish my list of top 2014 breakouts, I used a list of all batters with at least 400 plate appearances during the season. That list was then limited to batters who also had at least 500 plate appearances prior to 2014 to establish some level of a track record. A wRC+ differential was calculated for players, measuring their 2014 offensive performance versus their prior career average. The five largest differentials were selected as my "breakouts."

This list seems to pass the eye test, as these five players were each considered breakout stars last season, and all but the debatably snubbed Martinez earned their first All-Star game honors after multiple seasons in the big leagues. Let’s examine each a little more closely.

Devin Mesoraco

After a torrid 2010 season across three minor league levels, Reds prospect Devin Mesoraco appeared in multiple top prospect lists as a strong hitting catcher. However, in three partial seasons from 2011-2013, he produced only a 70 wRC+ at a near replacement (0.1 fWAR) rate over 589 plate appearances. While swinging more than average at outside pitches (35.9 percent), Mesoraco maintained league-average contact rates and above-average strikeout rates. This implied a problem of weak contact, which when combined with a lack of speed resulted in low BABIPs on both ground balls and fly balls.

In 2014, Mesoraco actually swung less often against all pitches (47.8 percent swing rate), but this new tendency resulted in a much lower 71.6 percent total contact rate versus the previous period’s 80.6 percent. However, he found his power stroke and nearly doubled his previous career ISO (.260). It resulted in a 147 wRC+, and the 77-point margin of improvement was actually larger than his prior career rate (70 wRC+).

2015 Status

Devin Mesoraco has battled a hip injury that may be ending his season before it gets off the ground. He hasn’t caught since April 12 and has seen very sporadic action as a pinch-hitter and DH since then. As hip rotation is a significant part of a swing, it’s understandable that his offense has been completely diminished since suffering the injury. In total, Mesoraco has seen only 51 plate appearances to a 48 wRC+ and may be done for the season, pending a decision on surgery to correct the problem.

J.D. Martinez

J.D. Martinez came up through the Astros’ farm system, hitting well at every minor league level before his call-up in 2011. He saw 975 PA between that season and 2013, producing only an 87 wRC+ with a high strikeout rate (23.7 percent) before being passed by every team in baseball in the Rule 5 draft. The Astros released him in the 2014 offseason despite much being written about his revamped swing, and the Detroit Tigers took a chance on him.

It certainly paid off, as Martinez’s single season wRC+ was 66 points higher than his pre-2014 career rate, and he produced 4.0 fWAR over 480 PA. He struck out even more than before, with a 15.3 percent swinging-strike rate, but his percentage of hard-hit balls dramatically increased (29.4 percent). He more than doubled his home run per fly ball rate from 2013 to 2014.

2015 Status

To this point, J.D. Martinez has managed to keep up his production with little regression. With a 128 wRC+ and a similar quality of contact profile to his previous season, Martinez is having another strong season. However, an even higher strikeout rate (28.6 percent) and high .257 ground ball BABIP do cause some reasons for concern. Still, with a revamped swing resulting in actual, observable differences in his performance, it makes sense that his 2014 season might not have been a fluke.

Josh Harrison

A former 6^th round pick by the Cubs in 2008 draft, Josh Harrison arrived to the Pirates as the third piece in a trade deadline deal for Josh Grabow and Tom Gorzelanny in 2009. Never a "name" prospect, Harrison’s defensive versatility, speed, and strong AAA performance brought him to the majors in 2011, and he saw intermittent playing time there until the 2014 season. From 2011-2013, he recorded a very pedestrian 78 wRC+ without much power and a small 2.6 percent walk rate.

In 2014, Harrison was potentially the most surprising breakout of this group. He assumed the Pirates’ starting 3B job over incumbent Pedro Alvarez and hit his way to a 137 wRC+ with stellar defense and speed while adding some pop to his bat (.175 ISO).

2015 Status

Of these five players, Harrison has most obviously regressed to this point in the 2015 season. He is currently on a very hot streak over his last 10 games, but on the whole, he still features a 91 wRC+, career high 15.2 percent strikeout rate, and is swinging more often than ever before (55.8 percent swing rate, 41.0 percent O-swing rate). Additionally, his power has regressed (.148 ISO). One encouraging sign is that Harrison is hitting an unusually high 34.6 percent of balls the opposite way, potentially indicating he is changing his approach.

Michael Brantley

Before 2014, Michael Brantley was the most consistent major leaguer of the players on this list. From his debut in 2009 through the 2013 season, Brantley was an athletic outfielder for the Indians, featuring speed and some of the league’s best patience at the plate. He featured minuscule 23.5 percent / 57.7 percent O-swing / Z-swing rates and made great contact with 81.1 percent / 96.1 percent O-contact / Z-contact rates. His power was weak (.105 ISO), and in total he was a roughly league-average hitter (97 wRC+).

In 2014, Brantley put it all together. He maintained his impressive plate discipline but added to it 8.2% more hard-hit balls, some decent power, and speed. Hitting 20 HR and stealing 23 bases, he became an all-around great player while producing a 155 wRC+ and 6.3 fWAR.

2015 Status

Brantley has continued his torrid pace into 2015, with a 154 wRC+ and again featuring his trademark plate discipline. His contact rates are the best of his career, his speed is still in play, and to this point he has demonstrated the most power of his career as well (.193 ISO). There isn’t really even a BABIP concern, as his .315 BABIP is fairly close to league-average. Brantley looks sustainable.

Anthony Rizzo

Anthony Rizzo was the biggest name on this list in terms of prospect potential and expectations. Rizzo was a multi-year top 100 prospect, and Cubs GM Jed Hoyer actually acquired him two separate times in two years – once from the Red Sox as the GM of the Padres and the next season from the Padres as the GM of the Cubs. Prior to 2014, he was a roughly league-average bat from 1B, sporting a 102 wRC+ with fairly high walk and strikeout rates while hitting for power (.174 ISO).

In 2014, the difference came as Rizzo hit his power potential at age 24. His .240 ISO was among the top 10 in baseball, and he turned in a 153 wRC+ and 5.6 fWAR season. His walks boosted his production, supporting his first 30+ HR season.

2015 Status

Rizzo has emerged as one of the top young hitters in baseball at age 25. He actually has walked more often than he strikes out (12.5 percent walk rate, 10.9 percent strikeout rate), maintained a .245 ISO, and added an odd penchant for leading baseball in hits by pitch (12). He has further increased his offensive production by 21 percent (174 wRC+) and looks on pace for another All-Star season.

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Spencer Bingol is a Contributor at Beyond the Box Score. You can follow him on Twitter at @SpencerBingol.