Another busy week so I'm going to keep it brief again. I am planning on writing up a playoff-oriented post for next week, however!
Today, we're going to take a quick look at JD Martinez. However, we're going to focus on the past 4 years for JD Martinez and idenfity a meaningful source of improvement.
In 2014, JD Martinez revamped his swing, quickly leading to 4.0 fWAR season complete with a 154 wRC+. After 2014, JD Martinez has posted 136, 142, and 165 wRC+s so he has established himself as one of the premier hitters in the MLB. As you likely know, Martinez's improvement banked a switch from being a line drive hitter to being a flyball hitter (swinging up on the ball instead of down on the ball).
If there was a flaw to be had about JD Martinez from 2014 until now, it was that he still struck out a lot and didn't walk a ton. However, this also happens to be the area that JD Martinez has been steadily improving upon:
Prior to his stint in AZ, JD Martinez was doing exactly what you wanted as a hitter - stop swinging at balls and make more contact with strikes. From 2014 through the first half of 2017, JD Martinez made significant improvement in both of these areas, which lead to a drop in his SwStr% from 15.2% in 2014 to 12.8% in the first half of 2017. These improvements have both led to a direct improvement in his K% - and as someone that makes some of the hardest contact in the MLB, more contact means more extra base hits.
However, things completely change here in Arizona. If his numbers in Arizona were applied to a whole season, they would be career highs in SwStr%/K% and O-Swing% and a career low in Z-contact%. If it weren't for his crazy 36.5% HR/FB% and .439 ISO, we might all be complaining about JD Martinez right now.
This is speculative, but you have to consider that much of this sample if AZ is due to an adjustment period from the AL to the NL. New teammates, new atmospheres, new ballparks (batters' eyes), new pitchers, playoff pressure, etc. These are all vary real but intangible things that have an effect on batters that we cannot effectively measure.
As an analyst, we should always be regressing to the mean. We have 3.5 years of trending data showing significant improvements to his plate O-Swing% and Z-Contact% (and, therefore, his SwStr%) and a huge outlier in 2017 in a much smaller sample. Going forward, he's much more likely to perform at a level around his 2016 or 2017_DET levels, which would be a huge plate discipline improvement for him in 2018.
This would be huge, because there is another thing that has changed for JD Martinez:
This season, JD Martinez has also shown a massive improvement in Hard%. Since being acquired by the Arizona Diamondbacks, JD Martinez has a higher Hard% than anyone in baseball - a full 1.7% higher than second place Joey Gallo. His full season Hard% (48.7%) is a whopping 2.8% higher than second place Joey Gallo (45.9%).
And, yes, again, this is something that you have to regress to the mean - but this is also something that is harder to fake. In small samples, it's easier to swing and miss more often than normal. In small samples, it's not really possible to hit the ball harder than you have before unless you've improved your talent. And what's also noteworthy is that he was showing a sign of this improvement in Detroit as well, so it's not just a matter of his adjustment at the time of the trade.
I don't think I've EVER condoned signing a soon-to-be 31-year-old free agent, let alone an outfield-first corner outfielder with poor defense, but JD Martinez is going to be an exception. He has shown the exact improvements needs to extend one's career as a hitter (good plate discipline lets a hitter survive longer as he ages) while also showing the ability to make harder contact than pretty much anyone else in baseball.
If JD Martinez's slugging is real (I think it is) and his plate discipline regresses back towards his prior-to-AZ levels, then JD Martinez is going to be looking at MVP-caliber offensive seasons for the next few years. If his interest in returning to AZ is true and the team can someone sign him at a bit of a discount, they need to do it.
Figure everything else out later, JD Martinez is the real deal.