Boston Red Sox shouldn’t invest too heavily in defense-first Jackie Bradley Jr.

If you’€™re a fan of Jackie Bradley Jr., and you should be, you’€™re probably thankful that his second half has been so good. Otherwise, all the Red Sox news (save Dave Dombrowski, and Travis Shaw’€™s Chris Shelton impression, I guess) has been a masterpiece of a disaster. A disasterpiece, if you will. But JBJ could make it all OK. Jackie has been hitting the living daylights out of the ball, especially for a glove-first center fielder. The young man has always flashed gap power, but in August he slugged .734, and raised his overall True Average to .299. In essence, he’€™s hitting about as well as Pablo Sandoval was supposed to.

But with Bradley, really any offense above league-average is gravy, as his calling card has been his ridiculous defense as the cornerstone of an outfield. With Mookie Betts playing well up the middle, Bradley has been plying his trade mostly in the land that Hanley and Manny tried to ruin and, as you might imagine, things are looking up. An outfield of Rusney Castillo, Betts and Bradley bears that "€œthree center fielders"€ tag that saberists love to hold up as an example of how teams can cheat an edge out of the competition, bringing up memories of the Seattle Mariners’€™ late-aughts defensive recipe and the Atlanta Braves running the legendary Michael Bourn-Jason Heyward-Martin Prado outfield out there for like a year and a half.

Here’€™s the thing, though: Defense-first outfielders can be dicey. I’€™m not trying to say that Bradley Jr. is a risky player or is going to be an issue going forward, but I’€™d like for Sox fans to temper their excitement with a dash of reason and a brief warning: This could go south in a hurry. And for the best example of how, you might want to look at a player on a team the Sox just beat twice last weekend: the New York Mets. Their former starting center fielder, Juan Lagares, could provide a bit of a cautionary tale.

This time last year, the Mets were in a similar position to the Red Sox. They had just wrapped up an August in which hope seemed lost for any sort of regular-season success. And yet, there were inklings that the future could be rosy thanks to guys like Jacob deGrom and especially Lagares. Lagares, a former prospect who profiled as a corner outfielder after converting from shortstop, suddenly arrived in Citi Field with a new M.O.: catching every flyball encroaching on Queens airspace. Lagares quickly turned into a no-doubt Gold Glover and also picked up the Fielding Bible Award for best defensive center fielder in 2014.

Now, Lagares was no great shakes at the plate in 2014, though he did show a marked improvement over years past. He hit well enough to skirt league average, with a .276 TAv and a respectable .321 OBP. FRAA, Baseball Prospectus’ defensive metric, gauged him out as only 8.0 runs to the positive, but his defensive numbers elsewhere were better than Ken Griffey Jr. dressing up like his own baseball card. UZR and DRS, two of the other major defensive metrics, gauged him worth about two wins on defense.

The Mets loved his 2013 and 2014 so much, they tossed a five-year extension and $23 million his way. Things were looking pretty grand.

This year? Different story. There are a couple of reasons why Lagares’€™ defensive numbers could have tanked this year –€” Jeff Sullivan takes his usually sharp look at them over at FanGraphs –€” but I’€™m not sure anyone argues that beyond the statistics, Lagares has looked different in center this year. He looks more mortal. Sure, defensive statistics are a bit unreliable in small samples and, sure, those samples could be more than a year long. But Lagares also passed the eye test in previous years, and he doesn’t pass the eye test nearly as well this season. He looks like a rib and elbow injury might be slowing him down, but whether it’€™s rumored weight issues, or an injury, or something else entirely, he might just not be the same defensive player he was prior to 2015.

So, that’€™s a long walk back to Jackie Bradley Jr. Like Lagares, Bradley is a defense-first player with real questions about his long-term offensive potential. Unlike Lagares, Bradley used to be a premium prospect, and he’€™s flashed more power than Lagares ever has, so he’s a different player, to be sure. But like Lagares, Bradley is likely to live and die based on his glove, not based on his lumber.

Also like Lagares, Bradley doesn’€™t come by his defensive chops based on blazing speed. Bradley is a great center fielder because he runs great routes and plays smart fundamental baseball. Sure, he’€™s got athleticism, but that undersells his instincts and his skill level. He doesn’t just run fast, he plays his position like a pro … just like Lagares did. And while that –€” to me at least –€” makes it seem a little more likely his skills will diminish slow(er) with age, it doesn’™t make skill drain bulletproof.

Bradley still requires athleticism of some kind to provide the dynamic range that makes him an above-average big league regular. An injury could drain that speed and turn a great defensive outfielder into an average one –€” and average defensive outfielders need to hit pretty well to hold a spot. An arm injury could turn a plus-plus arm into a good one.

Lagares isn’€™t the only plus-plus defensive outfielder to be robbed of his generational talents and become something much less. Franklin Gutierrez and Michael Bourn both headlined breathtaking outfield defenses and posted other-wordly numbers in center. But both also tailed because of health and/or loss of speed. Defense is a young man’€™s game, and time can take its toll. The critical thing for a sharp-gloved center fielder is to hold onto those talents for as long as possible, and supplement those defensive chops with some solid hitting for when that second skillset starts to slip.

Today, that’€™s not a concern for Bradley.

As I mentioned earlier, I don’€™t think Bradley is a serious risk to start misplaying balls in the middle of Fenway –€” at least not any time soon. Beyond that, his recent offensive outburst has shown there’€™s a glimmer of a serious offensive player in him as well … even if it’€™s likely that he’€™ll regress that slugging percentage back under .700 in short order.

Great defensive center fielders are terrific to watch, one of the true aesthetic joys of the game, and Bradley already seems poised to take a place among the best of this decade. Just be warned, if you rely too heavily on defense at his position, there’€™s a chance you could end up more as a cautionary tale, pushed aside at a critical point to make room for a bigger, badder bat.

Bradley is doing his best to cement his reputation as an all-around star, not just a defense-first guy. That would do quite a bit to lower his risk, and make the Sox very happy to invest in him long-term.