Center fielders don't seem to age well, especially those who derive a significant amount of their production from their gloves.

And yet [baseball-trade-values-player-link player="8624"]is still a capable major league player for the Tampa Bay Rays. So are [baseball-trade-values-player-link player="7701"] and [baseball-trade-values-player-link player="9143"] of the Colorado Rockies and Los Angeles Dodgers, respectively.

So why do they have negative trade values?

First, it's important to reiterate that here at BTV, we aren't in the business of evaluating talent. Our values are derived from publicly available projections and figures, and include a standard aging curve. So the explanation seems pretty clear - all three players are signed to significant contracts, and as they age, they are projected to produce less value than they are owed.

But these players are part of a larger trend. In their primes, all three outfielders derived a significant portion of their value from their defense in center field and their speed on the basepaths. As they've aged, these skills have deteriorated and their offensive production hasn't been enough to sustain positive value.

At only 29, Kiermaier is the youngest of the three, and also remains the best defender. But in 2019, he posted a career-low 13 Defensive Runs Saved and the second worst Ultimate Zone Rating of his career. He has also had trouble staying on the field, as his high intensity in center has led to various injuries over the past few seasons. As he ages, he's looking more and more like a non-elite defender that has trouble staying on the field and can't be expected for more than an 80 wRC+. That type of player isn't worth the $36 million he's owed over the next three years, hence his -$13.6M valuation. And if he loses another step in the outfield, he could become even more of a sunk cost.

The 2019 season was Pollock's first in Los Angeles, and he missed almost half of the year due to ongoing elbow issues that he has been battling since the beginning of 2016. He's also lost a foot per second in sprint speed since 2015, explaining his drop from 39 steals that season to only five last year. These two issues have combined to make Pollock a well below average defender, as he posted -10 DRS, a -18.9 UZR/150 and -7 Outs Above Average. His bat has been slightly above average over the past three seasons, but combined with his declining defense and baserunning, that's not enough to be worth the $56 million he's owed through 2023. That's why his trade value stands at -$41.3M.

Blackmon presents the best case scenario for this type of player. While he was never an elite defender, Blackmon's defense and baserunning have declined similarly to Pollock's. His sprint speed has dropped from 27.9 ft/sec in 2015 to 26.7 ft/sec in 2019, tanking his stolen base totals from 43 to 2 in that span. The Rockies can't afford a slow, poor defender in center at spacious Coors Field, so after Blackmon posted -28 DRS at the position in 2018 he was moved to right field. He's still been a below average defender in the corner, but his bat (20% above league average the past two years) makes up for it and leaves him as an average regular. He isn't worth the $74 million he'll be paid over the next four years, but at -$16.0M in trade value, he isn't killing the team.

• These three aren't isolated cases. Instead, they should serve as cautionary tales. There are a handful of similar outfielders available on the free agent and trade markets who may be susceptible to similar declines. 

[baseball-trade-values-player-link player="7737"], Red Sox: Arguably the best available center fielder this offseason, Bradley Jr. fits the profile we're looking for perfectly. He'll turn 30 during the 2020 season and he's already seeing his formerly elite defense decline, having posted negative DRS and UZR/150 values last season for the first time since his rookie year. He's settled in comfortably as a 90 wRC+ bat, and if his defense continues in this direction, he won't be an attractive corner outfield option.

[baseball-trade-values-player-link player="8804"], Pirates: Marte is almost the same exact story. He's been a solid-to-great defender in center for his whole career, but dipped into the negatives for the first time in 2019. In Marte's favor, his bat is still a plus and he's still one of the better runners in the game. At 31, he might see a Blackmon-like transition into a corner outfield spot within the next couple years.

[baseball-trade-values-player-link player="9125"], Giants: A non-tender candidate for the Giants, Pillar has been in a steady decline since his best defensive year in 2016. He was a slightly below average defender in center in both 2018 and 2019, and while he was solid in right field last year, his bat is nowhere near good enough to remain an everyday player in a corner.

[baseball-trade-values-player-link player="8237"], FA: Gardner is an odd candidate for this list, as he had arguably the best offensive season of his career in 2019. This was thanks in large part to a career-high 28 home runs, likely aided by the juiced ball. He isn't getting younger, and a switch back to a calmer baseball could hurt him. The Yankees have used the 36-year-old in all three outfield spots last season, but he posted a negative DRS in center for the first time in his career. He's more of a corner guy going forward, and if his bat takes even a small step back, he might be in trouble.

[baseball-trade-values-player-link player="8836"], FA: Another Yankee who had a career offensive year in 2019, Maybin could be seen as a sneaky gamble in a thin outfield market. But he was a negative defender in left field last season, where he saw the majority of his playing time. He still has some speed, but it's nowhere near what it used to be. His 127 wRC+ season came out of nowhere, and if he can't sustain it, his glove doesn't give him much room to remain a valuable everyday player.

Shogo Akiyama, FA: As an international free agent from Japan's NPB, Akiyama is a wild card, and we don't know a ton about him. But from what we do know, a large part of his game centers around his speed and his plus center field defense. A team trying to sign Akiyama will already be taking a gamble on his bat translating from the NPB to MLB, but he'll be 32 in 2020, making his defense a question mark as well.

This isn't to say that any - or all - of these players will take a nosedive in 2020. In fact, I'd bet most (if not all) remain productive players for the next season or two.

But teams should hesitate before investing in any of these players long-term. Giving up significant trade assets for Marte or Bradley Jr. could backfire within a few seasons, as could offering Akiyama or Gardner multiple years on a free agent deal. On a similar note, the New York Mets and Minnesota Twins should think long and hard before trying to ink [baseball-trade-values-player-link player="7790"] and [baseball-trade-values-player-link player="8980"] to lucrative long-term extensions.

In today's game, youth is more important than ever. That's especially true up the middle. Shifting has allowed teams to leave an aging defender at shortstop, but a center fielder will always need to be able to cover ground to remain at the position. And if a center fielder doesn't have the bat to support their inevitable move to a corner, they could end up becoming a sunk cost.