On WFAN Tuesday afternoon, New York Mets general manager Sandy Alderson sang the praises of his young center fielder, Juan Lagares. And why not?

Per Baseball-Reference.com's wins above replacement, Juan Lagares has been worth 3.6 wins already this season. That's the eighth-best mark in the major leagues among center fielders, and the seven center fielders ahead of Lagares have played many more than his 97 games, as have most of those on the list below him.

That's because Lagares didn't debut until the end of April, and didn't earn the full-time center field job until June. The Mets turned to him out of desperation, not believing he was ready, but he's been sufficiently valuable that only David Wright, who was having another of his Hall of Fame-level seasons, and the inexplicable Marlon Byrd were more valuable for the 2013 Mets this season.

Still, the way Mike Francesa phrased his question to Alderson is telling: "Do you consider Lagares to be a guy who is part of your future?"

There's still that question, not just in the mind of Mike Francesa, and it's kind of amazing. Over the past ten years, ten players, total, have surpassed Lagares' WAR in their first season with a major league team. The list: Jason Heyward, Bryce Harper, Austin Jackson, Evan Longoria, Yasiel Puig, Mike Aviles, Denard Span, Nolan Arenado, Yoenis Cepedes and Dustin Ackley. And if Lagares keeps playing like he has for the final month, he's a good bet to pass Ackley and Cepedes.

No one was asking at the conclusion of the first year for any of those players whether or not they were part of their teams' futures. So why, then, are some people unwilling to accept Juan Lagares as a legitimate building block for the Mets?

There's a pretty simple answer: it's all about defense and the unknowns for those metrics when it comes to building a lineup.

Juan Lagares has created 2.8 of his 3.6 WAR through his otherworldly glove. This isn't the only metric that has him so high. ESPN just named his defensive player for the month in August, with 12 defensive runs saved, and one out-of-zone play made every 9.9 innings, best mark at the position of center field.

But it's all measuring the same thing, really, which is that Lagares is a bit above replacement level as a hitter, with ample warning signs in his offensive line, from his low walk total to a lack of power to an awfully high strikeout rate. It's his defense that makes him a building block.

Just under 78 percent of his WAR comes from defense. Among the seven center fielders ahead of him in 2013, no one is close to that dependent on defense for value. Only Carlos Gomez even approaches getting half his value from defense, and among those below him, Lorenzo Cain of the Royals is close, but a bit less dependent.

But let's take a closer look at Cain, because it gets to the central question of just how reliable defense can be, year-over-year, at this point. Cain was considered a good defensive center fielder last year. His UZR/150 in 2012 was 23.2 runs above average. In 2013, it is 23.9. He's just playing more in center field.

Craig Gentry of the Texas Rangers is another recent example. Last season, among players with at least 600 innings in the field at any position, his 30.6 UZR/150 led baseball. This season, he's played less, but his UZR/150 is a very comparable 27.8.

But there are counterexamples, too. Last month, the New York Post's Joel Sherman wrote a piece lauding the Mets for not landing Michael Bourn this past winter, since it allowed them to play a better, cheaper option in Lagares. Bourn's UZR/150 last season was 23.2. This season, he's barely above average, checking in at 2.5. Chris Young saw an even steeper drop, going from 18.7 last season with the Diamondbacks in center field, to -20.2 this season with the Athletics.

But teams employ, and even build around, one-dimensional players all the time. What seems to be different is how confident a team is that the guy who hit 30 home runs last season will hit around 30 home runs next season, and how confident teams are that the guy who saved 23 runs in center field last season will save 23 runs next season. There's the double risk, not just that a guy so dependent on one part of his game could regress, but also that we might be measuring it inaccurately. No one disputes that a home run is a home run, after all.

There's essentially only one question for the Mets to answer when it comes to Juan Lagares. One is whether they think his defense in 2013 is for real. If it is, finding a four-win center fielder on the open market is no easy task, even if price were no object (and rest assured, price is most certainly still an object for Fred Wilpon's Mets).  

If Lagares' defense is anything like it's been in 2013 moving forward, then there's nothing else to figure out, really, other than how to develop greater certainty about building with defense, especially in a league that is trending toward fewer runs scored with each passing year, making the saving of 30 runs more valuable than it was five years ago.