Comparing Alek Thomas to Adam Eaton: A Cautionary Tale

Should we be more concerned about Thomas' ground ball rates or the Diamondbacks selling low?

Throughout the course of Alek Thomas' trip through the high minors and his rookie MLB season of 2022 I had a nagging feeling that his high ground ball rates were going to hold him back. As Thomas began to slump over the final couple of months in 2022 prior to being sent back down to Triple-A Reno I confess to some confirmation bias kicking in. After all his ground ball percentage of 58% was the third highest in the league. 

But another nagging feeling began to gnaw at me. Didn't I have a similar concern about another young Diamondback left handed outfielder almost 10 years earlier? Despite excellent minor league batting numbers, that player had a high ground ball rate. Then over his first nearly 400 MLB PA he had a 58% ground ball rate contributing to a disappointing sophomore season. (League average GB% is usually between 43-45%) I was concerned at that time the batted ball profile was coming home to roost. 

Of course I'm talking about Adam Eaton. It was during the 2013-14 offseason, flush with left handed hitting outfielders on the roster and in the system the D-backs were  in search of more right handed power, (sound familiar?) The team proceeded to move Eaton to the Chicago White Sox in a multi team trade getting Mark Trumbo in return. All Eaton did over the next three years was bat .290/.362/.422,  .783 OPS, or 119 OPS+. Coupled with +23 defensive runs saved he was worth 16 WAR, or 5.3 WAR per season from 2014-2016.  And he did that despite a ground ball rate of 54% during those three years. Oops. 

No two players are exactly alike, and comparisons, while interesting and sometimes informative, have predictive limitations. Nonetheless "comps" are a staple of both the scouting and analytic communities. So I decided to try to tease out some comparable data to see how these two players actually compared, beyond my cursory glance through their stat pages and focus on ground ball rates. 

A few key points to keep in mind. Eaton was drafted out of college and made his pro debut at age 21. Thomas was drafted out of high school and made his pro debut at age 18. That three year difference in age is significant of course, as is the fact that Thomas was deprived of a competitive 2020 minor league season due to the Pandemic. But the age gap narrows as we get to the higher minors. 

The other things to note are the stats chosen to compare. These are stats I often focus on when evaluating minor league hitters. 

wRC+ is weight runs created metric that is adjusted for league average.  League average is 100. The higher the number  above  100, the greater the percentage above league average the player was. It's important to look at a player in the context of the league and ballparks he's playing in. wRC+ allows us to do that and not be mis lead by raw rate stats such as batting average, slugging, etc. 

ISO, or Isolated Power is simply Slugging %  minus Batting Average. Sometimes a player without a lot of raw power but with  a very high batting average will have a high slugging percentage. ISO allows us to then isolate a player's power production. It's an important tool to use in evaluating minor league players to help tease out the true power potential they have, beyond just looking at homer totals or slugging percentage. 

BB/K ratio is self explanatory. Understanding a player's plate discipline profile and batting eye are critical to evaluation. 

GB% is included because that is what drew me to the comparison in the first place. 

We start with their entry into pro ball at the Rookie league level. At this stage Eaton being older comes into play. He's got a much higher wRC+, a much higher ISO, and better plate discipline.  The ground ball rate is the same however. 

As we move up to the next levels of the minor, A & Advanced class A+, the gap between the players narrows quite a bit. In fact Thomas has almost caught up in wRC+ and ISO but plate discipline starts to be come a big difference. 

By the time we get to the AA comparison, the age gap has been cut by one year due to 2020 missed season for Thomas. The overall production above average is exactly the same, 134 wRC+, but they got there in very different ways. Thomas was hitting for much more power, while Eaton was hitting for much more batting average and OBP. Keep in mind however Thomas was playing in Amarillo, the most homer friendly park in all of baseball, while Eaton was in a pitchers park in a pitchers league. (Mobile in the Southern League)

The trend continues at AAA. The wRC+ is similar, Thomas has much higher ISO and power, but Eaton has much higher batting average and on base percentage, continuing to show good plate discipline throughout his minor league career. 

Eaton actually had a very successful first cup of coffee at age 23 in 103 PA, putting up a 119 wRC+. He battled an elbow injury in the early part of 2013, and didn't come back up to MLB until July. He struggled to an 84 wRC+ as his walk rates, on base, and power dipped across the board.  Thomas got off to a hot start to his rookie season as well, posting a 110 wRC+ through his first 107 PA before seeing his numbers dip

Summary Conclusion:

There a lot of areas of similarity. The overall production, wRC+ is very close once they get out of rookie ball. And their batted ball profiles are very similar. But these are two different players and Thomas will find his own path. In general, I would say that Thomas has shown more power potential, but will likely never have the kind of plate discipline that Eaton displayed throughout his career. It's also possible that Thomas' perceived higher power potential is a function of changes in the ball and batter approaches we've seen the last six or seven years. 

Looking at how Eaton broke out to be an all star caliber in the three years immediately following the trade, I chastise myself a bit for ignoring previous lessons learned. I also think back to a Mike Hazen comment at the winter meetings about trading away young players, and hope he continues to listen to his inner voice as well. 

"When you go to make that trade everybody is always worrying about the worst case scenario. Which is my player turns into a superstar and I'm getting a risky young player on the other team that doesn't turn into anything.......you wonder if you trade the guy who is a 1+ who hasn't hit yet when we all know the landscape of MLB players. In the first 150-200 plate appearances, what's the average performance of those young players?There's not many that perform very well.  I remember Dustin Pedroia in September of 2006. He didn't hit at all. If you had made a decision based on that one month you'd have been dead wrong"