J.D. Martinez continues to be one of the most sought-after bats on the trade market, and for good reason. In three years with Detroit, he's hit 83 home runs with a line of .299/.357/.540 (143 wRC+) that's made him the batting equal of Edwin Encarnacion (.269/.361/.544, 144 wRC+) and Giancarlo Stanton (.266/.361/.544, 144 wRC+) in that time. With just one year left on his contract at $11.75 million, Martinez also won't require a massive long-term commitment, so it's not hard to see why teams like the Mets (before Yoenis Céspedes), Giants, and Dodgers showed interest.
But any team acquiring him -- particularly National League teams -- must first answer a pretty important question: How much of that offensive value does Martinez give back on defense?
That's an extremely big deal, because both FanGraphs and Baseball Reference agreed that Martinez dropped from a five-win player in 2015 (i.e., star level) to a two-win player in '16 (i.e., league average), despite the fact that his batting line improved (from .282/.344/.535, 137 wRC+ in '15 to .307/.373/.535, 142 wRC+ in '16). How is that possible?
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The minor reason is simply that he didn't play as much in 2016, missing nearly seven weeks with a fractured right elbow, leaving him unavailable to accumulate value in a counting stat such as WAR. The larger reason is that the major defensive metrics graded him far less favorably, whether you're looking at Defensive Runs Saved (from +4 to -22) or Ultimate Zone Rating (+7 to -21). The rule of thumb is that 10 runs is equal to one win, so, a drop of more than two fielding wins would account for just about all of his total drop in value.
That's the difference between "slightly above-average on defense" and "among the worst in the game," and teams are going to need to know which best reflects Martinez's skills moving forward. The correct answer is probably somewhere in between, because single-year defensive stats are problematic at best and misleading at worst, but which is closer to being right? Let's break down what goes into that -21 UZR (range, arm, and errors) and throw in some Statcast™ to see if we can get more clarity.

Range
-12 runs (from -2 in 2015)
While you need to touch a ball to make an error, range accounts for the fact that you can miss a playable ball because you didn't get to it. By this measure, Martinez was considered roughly average in 2015, and well below-average in '16. 
Fair or not? Let's start with a simplistic look at non-homer, non-grounder batted balls between 200-400 feet to right field, which is a high-level look at the kind of balls that a fielder could theoretically get to. While unscientific, it's a decent start, and Detroit right fielders (mostly Martinez, some Steven Moya and Mike Aviles) allowed a .452 average on those balls, the third-highest in baseball. The two best teams were the White Sox (Adam Eaton, .339) and Red Sox (Mookie Betts, .350); the two worst were Colorado (huge outfield, .463) and Baltimore (Mark Trumbo, .457), so that passes the smell test. In 2015, the Tigers were about league average.
So that indicates Martinez's range did decrease. What if we look at a Statcast™ visual of batted balls that were likely to be caught? By examining the hang time of each batted ball and the distance that an outfielder was projected to have to cover, we can put an expected catch rate across the entire Major Leagues to each ball. If we look at batted balls that were 75 percent likely and above to have been caught, we can see that there's more than a dozen for Martinez.

For comparison, here's Jason Heyward, who played just about the exact same amount of time (1027 1/3 innings) in right as Martinez (1029 1/3) did, and you'll see far fewer. 

No one expects Martinez to field like Heyward does, of course, and it appears balls near the wall were a particular issue. Eyeballing it this way, it does seem that at least some of the poor range grade in 2016 was real.
Errors
-2 runs (from 0 in 2015)
This is defined as "Does the player commit more or fewer errors compared with a league-average player at their position? And while errors are an extremely flawed stat, Martinez did go from 2 in 2015 to 6 in '16.

Still, this was a very small input into his total number, and it doesn't seem to be much to worry about going forward, so we'll move past it quickly.
Arm
-3 runs (from +8 in 2015)
The obvious caveat here is, "Hey, the guy broke his throwing elbow!" And that's fair, because Martinez' arm was considered a positive in 2015, and we see a drop of -11 runs here. While his overall "competitive arm strength" didn't drop that much from 2015 (91.1 mph) to '16 (90.9 mph), there's definitely a difference within '16 before the arm injury (91.9 mph) and after (89.4 mph).
So that could easily explain part of it, because he went from a fielder with a good arm and average range in 2015 to a fielder with a poor arm and poor range in '16 -- a bad combo! But while you might be immediately looking at his drop from 15 assists to three, remember that a good arm is about more than just throwing runners out. After all, you wouldn't want to punish good outfielders for scaring runners into not trying to advance at all.
Still, if you look at some of the situational numbers on Baseball-Reference, the drop is clear. Martinez held more than 52 percent of runners from advancing on him in 2015, either via assists or by them not attempting to advance. That dropped to 38 percent in '16. If you take singles with a runner on second, he held only 27 percent in '16, after 59 percent in '15.
Why? Well, the reduced arm strength could be a reason. Teams knowing he had injured his arm and potentially being more aggressive could be another. And the fact that Martinez was playing deeper this year (289 feet from home, after being 282 feet in '15) could be another.
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So what's the takeaway from all this? The defensive issues that plagued Martinez in 2016, particularly in terms of range, seemed to be real; this was not a case of unfair metrics. But for a quick comparison, you could take Nelson Cruz, who had a similar batting season with more playing time and slightly less objectionable defensive value, and had a four-win season. That's maybe the best possible outcome for Martinez -- and while it's not a bad one, he's probably not going to be a five-win player again.