Nicholas Castellanos has a decent claim to make to be considered the top free agent outfielder available this winter, but that description hinges on one very important question: Is he actually an outfielder?

It's a question worth diving into, because Castellanos' defensive metrics haven't been kind, to put it mildly, and a DH-only profile immediately cuts his market in half. We probably saw this exact thing happen with another Scott Boras client, J.D. Martinez, earlier this offseason. After Boras vociferously claimed that Martinez wasn't just a DH, Martinez ended up not opting out of his contract -- likely in part due to well-founded fears that no National League team would consider him if he had to play the field every day.

Considering that Martinez is a far superior hitter to Castellanos, this is a pretty serious aspect of Castellanos' free-agent case. Then again, he's also five years younger than Martinez, and there's at least a few pieces of evidence that point to him being a competent, if not exactly stellar, outfielder for the next few years. (By that time, the NL might have the DH anyway, right?)

If that's true, then NL teams should have plenty of interest in Castellanos. We think it's true, and reportedly the Marlins are expressing interest. Let's explain.

1) Castellanos has not been a good defensive player to date
This is unarguable, and so we have to start here. Castellanos was Detroit's regular third baseman for four seasons (2014-17), their regular right fielder in '18, and then played 137 more games in right (with 11 additional in left) for the Tigers and Cubs in '19.

It ... hasn't gone well. Some of this predates Statcast, so we'll start with Defensive Runs Saved. Each of the four years at third base was cover-your-eyes bad.

2014 // minus-30 DRS
2015 // minus-9 DRS
2016 // minus-11 DRS
2017 // minus-14 DRS

During those four years, he was the weakest defensive third baseman in baseball (minus-64) by more than double the second-place minus-31 of Yunel Escobar, though part of that is playing time; most fielders with those numbers don't continue to play there. Near the end of 2017, Castellanos moved to right, and he somehow piled up minus-7 DRS in just 21 games that September.

In 2018, playing exclusively in the outfield, he had minus-19 DRS (fifth-weakest in the Majors) and minus-24 Outs Above Average (or OAA, Statcast's defensive metric), which was the weakest of any outfielder. Those are mild disagreements, because the consensus was: not good. From 2014-18, Castellanos was the second-weakest fielder at any position. It's unsurprising, then, that this past June, we received word that the Tigers had discussed the idea of moving him to first base. Why wouldn't they?

2) But 2019 was a step in the right direction
OK, so, that's all bad. That's a lot of value lost. That's why, despite Castellanos being an above-average hitter in his first five seasons (.274/.323/.460, a line 10% better than average), he was worth only 7.8 Wins Above Replacement. Mike Trout just had 8.6 WAR in 2019 alone, and a mark of 2 WAR in a season is considered "average," so the value Castellanos provided with his bat was mostly given back by his glove.

This past season, however, was better. Castellanos went from minus-24 OAA to minus-7; while that's still a negative, it's a sizable jump. He went from minus-19 DRS to minus-9; again, still not great, but improving, and playable. It started in the first weekend of the season, when Castellanos covered 108 feet in 5.4 seconds to get to this Kevin Pillar popup:

This ball had a 60% Catch Probability; while that may sound high, and it is, it was also the second-most difficult play Castellanos has made as an outfielder. (His toughest, a 50% play, came on July 23.)

But it really hasn't been about making great plays, because he mostly hasn't. It's been about cutting out the mistakes. In 2018, Castellanos failed to collect 17 plays that had a Catch Probability of 80% or higher -- for example, this one:

In 2019, he cut that number in half, to just 9 of those highly damaging misplays, or about 1 1/2 per month.

As you might have expected, he put in a ton of work last offseason trying to improve, as Tigers coach Dave Clark told the Detroit News in March.

“Everything about the kid in the outfield is better,” Clark said. “His first step is great, a lot better. He goes back on balls well. He’s reading the ball off the bat a heck of a lot better than he did when he started."

We can quantify some of that. While Castellanos' Jump (feet covered in the right direction in the first three seconds) is still below average, his reaction, or what Clark referred to as "first step," did get better. In 2018, his reaction time -- feet covered vs. average in the right direction in the first 1.5 seconds -- went from minus-0.8 to 0.0, or average.

It's not huge, but it's something, and it headed in the right direction.

3) The start to his season was actually pretty good
Here's what we tweeted on June 11:

That's right. Through the first half of June, Castellanos was a scratch fielder. No, "average" isn't a star, but it's an incredible improvement from where he'd been. So what happened? Breaking it down by month, we can see:

April // +1 OAA
May // 0 OAA
June // minus-4 OAA
July // minus-2 OAA
Aug. // minus-1 OAA
Sept. // minus-1 OAA

For the first two months, Castellanos was about average. For the last three months, he was slightly below average. But in June? For reasons we can't explain, other than "maybe fielding slumps sometimes too," he had a rough few weeks. On June 9, he couldn't come up with a catchable ball (60% Catch Probability) hit by Whit Merrifield. On June 19, he got a poor jump and was unable to get to an easy play (85% Catch Probability) off the bat of Elias Díaz.

And on June 15, against Jordan Luplow, on a ball that had a 99% Catch Probability, he did this:

That's not a good look, but then again, Castellanos has played a lot less outfield than you'd think. A lot less.

4) He's still surprisingly inexperienced in the outfield
Castellanos may or may not have the skills to play a strong outfield, but he also hasn't exactly been put in position to succeed there, either.

He was drafted as a shortstop, and played exclusively third base in his first full pro season in 2011, as well as for the first half of '12. That July, with Castellanos hitting well at Double-A and with Miguel Cabrera manning third for the Tigers, he was moved to right field. In 2013 at Triple-A, Castellanos then played exclusively left field. But after the season, the Tigers traded Prince Fielder and moved Cabrera to first, so when Opening Day 2014 rolled around, Castellanos was making his Major League debut at third base -- a position he hadn't played in a year and a half.

After nearly four years of struggling at the hot corner, he then went back to the outfield near the end of 2017. So after all this time, more than nine years since he was drafted, the entire sum of Castellanos' outfield experience looks like this:

Left field -- 1,241 innings (74 1/3 in the Majors)

Right field -- 3,011 1/3 innings (2,578 1/3 in the Majors)

That's a total of just over 4,250 pro outfield innings. By comparison, let's look at a few stars within a few months of the same age as Castellanos. (This is pro ball only, so not accounting for anything as an amateur; Castellanos never played outfield before Double-A.)

Mike Trout: 12,455 2/3 innings of pro outfield experience
Bryce Harper: 10,547 2/3 innings
Christian Yelich: 10,377 2/3 innings
Joc Pederson: 8,612 2/3 innings

You get the idea. Castellanos has said, repeatedly, that moving around so much has been difficult.

“Last year, everything was brand new,” he told Grantland in 2015 about playing third base. “All the fields were brand new. The players were brand new. How our pitching staff attacked hitters was brand new. I had to learn that all on the fly, while learning a new position. So the game definitely got fast for me a little bit over there sometimes.”

“I mean, it’s my second year out there,” he told the Athletic this past June about right field. “The more time I’m out there, the more natural it’s gonna become, the more my athleticism is gonna take over and allow me to play.”

"I just want to stick to right and try to be as good as I can in one spot," he said to MLB.com in June, referring to the possible move to first base. "That was the first time where I said no. I've said yes a lot. So last year, it was just like, 'No, not right now.'"

5) Is there any precedent for future improvement?
We've explained how poor Castellanos had been, how he improved in 2019, and some possible reasons why. But is there any more left? That is: Has anyone put up a season as poor as Castellanos did in 2018 and managed to eventually add value on defense? Since this predates Statcast, we'll use FanGraphs' "defense" metric, which is a position-adjusted version of UZR, going back to 2002.

We can narrow this down only to outfielders, and we're not interested in, say, 36-year-old Gary Sheffield's poor defense for the 2005 Yankees, or 34-year-old Manny Ramirez struggling for the 2006 Red Sox. Castellanos' youth is part of the sales pitch here, since he's not even 28 until March.

If we just look at outfielder seasons aged 29 or younger, we find that Castellanos' 2018 was the 24th-weakest of 633 seasons. That's actually 22 players -- a few qualified in more than one season -- and it gives us something to look at. How'd they end up doing?

As you'd expect, mostly poorly; offense-first players like Adam Dunn, Adam Lind, Jack Cust, and Dmitri Young would soon be done with the outfield. You're not looking to Jason Bay, Brad Hawpe, Carlos Quentin, or Jonny Gomes for hope. Cabrera pops up here as well; he was poor in 2004, worse in '05, and then he was an infielder. One player, Seattle's Domingo Santana, made the list this year, so we'll have to see how he progresses.

The best-case examples, sort of, are former Dodgers Andre Ethier and Matt Kemp, who somehow went from awful (the 2010 Dodgers outfield is actually the worst on record) to each winning Gold Gloves in '11. It's difficult to say that either actually deserved to win -- a discussion of how Gold Glove voting worked eight years ago is a discussion for another time, or more appropriately, never -- but hey, there's your best possible upside.

There's one name, however, that stands out as a good comparable: Ryan Braun. Like Castellanos, Braun was drafted as a shortstop and arrived in the Majors as a third baseman. Like Castellanos -- sooner, actually -- he quickly played his way off the position, moving immediately to left field after a 2007 rookie season that made it clear the hot corner wasn't for him. Other than a two-year trip to right field in 2014-15, he's been in left ever since.

Braun was surprisingly OK in 2008, but '09 was a complete debacle, which is how he ended up on our list. Different defensive metrics of the time will tell you somewhat different stories, but he's been playing regularly in the outfield for a decade now, even as he approaches his 36th birthday. He's never been great, really, but he's been competent enough to keep wearing a glove. (Coincidentally, a 2018 Castellanos miscue in right field helped give Braun a home run he wouldn't have otherwise had.)

That's probably the most reasonable best-case path here, which is both encouraging -- it can happen, long-term -- and discouraging. (Most of our comparables didn't last. Castellanos isn't the hitter Braun is.)

The larger question here might be what kind of hitter Castellanos is, because he's probably not as unimpressive as he was with Detroit this year (105 OPS+) nor as great as he was with Chicago (151 OPS+). Which end of that he turns to be closer to might say as much about his glove as to whether he lasts in the outfield, because you can live with below-average defense from a slugger in a way you wouldn't from an only-OK bat.

Castellanos, when he was with the Cubs, didn't kill his NL team on defense. He's not likely to add much value, because he simply doesn't have the speed or athleticism of Trout or Mookie Betts, but that's not the expectation. Only entering his age-28 season, with half as much experience as other outfielders his age, and with clear improvement in his second year in the outfield, there's reason for NL teams to be comfortable with him in their outfield next year. After all, it's not like the other major outfield free agent, Marcell Ozuna, fills you with defensive confidence of his own.