Here are the National League leaders in Wins Above Replacement, via Baseball-Reference.com:
Clayton Kershaw: 7.3
Jason Heyward: 6.3
Giancarlo Stanton: 6.1
Jonathan Lucroy: 5.7
Troy Tulowitzki: 5.5
And here the NL leaders in WAR via FanGraphs:
Clayton Kershaw: 5.9
Jonathan Lucroy: 5.6
Giancarlo Stanton: 5.5
Hunter Pence: 5.4
Jason Heyward: 5.2
Andrew McCutchen: 5.2
Kershaw leads both sites in WAR so the statistical consensus is that he's been the best player in the National League, even though he missed a month of action back in April. He's 16-3 with a 1.73 ERA, so while he may not get to 200 innings he's been so dominant that he still has the highest WAR.
But ... no National League pitcher has won the MVP Award since Bob Gibson in 1968, so Kershaw still has to overcome that bias against pitchers. Plus, he could slump in September and lose a couple games (unlikely, I know, since he's allowed more than three runs in a game just once, but I guess it could happen). Stanton's Marlins aren't going to make the playoffs, and MVPs usually come from playoff teams (see Miguel Cabrera versus Mike Trout). Lucroy has certainly been terrific, although lacks the big power and RBI numbers MVP voters usually favor, plus the Brewers aren't a lock to make the playoffs.
Enter Heyward, under-the-radar MVP candidate. Based on WAR, he's been one of the best all-around players in the league. Not that he's gotten recognition as such.
Of course, he has no chance of winning; in fact, I'd be surprised if he even finishes in the top-10 in the voting. He's hitting .272/.354/.391 with 11 home runs and 54 RBIs and right fielders slugging under .400 don't get MVP support. Heyward's value comes with his defense. Baseball-Reference uses Defensive Runs Saved for its defensive component of WAR and Heyward leads the majors with 33 runs saved above average. Only Juan Lagares of the Mets is at +30, and only four other players are at +20 or higher. FanGraphs uses Ultimate Zone Rating for its defensive component and Heyward leads all fielders there as well, at +26.4 (only Alex Gordon and Lagares are at +20 in UZR).
So those defensive metrics agree that Heyward has been the best defensive player in baseball and that he's saved a lot of runs. Those runs saved are worth about three wins -- so more than half of Heyward's value has come with his glove.
Yes, it's easy to dismiss one-year defensive numbers. Or perhaps wise to use them with caution. Last year, Carlos Gomez had 38 DRS and Gerardo Parra 36, and this year those players rate at 0 and +1, respectively.
But Heyward has always rated as a top defender -- not quite at his 2014 level, but he's averaged +21 DRS per 1,200 innings in his caree, compared to his rate of +34/1,200 innings in 2014. There's no reason to write off the metrics as a one-year anomaly.
What makes him so good? He doesn't have Roberto Clemente's arm (although he does have nine assists), but he has great instincts and range. Let's use an old-school fielding stat: Range Factor, which is simply putouts + assists per nine innings. Heyward has averaged 2.55 plays per nine innings compared to the league average of 2.06 for right fielders. Based on this simple math, he's made one extra out every two games compared to an average right fielder -- 0.49 per nine innings. He's played 1,157 innings so far in right field (128.5 games worth), so that's about 64 extra outs he's made above an average right fielder, let alone a subpar one.
Imagine if we added 64 hits to Heyward's résumé: He'd be hitting .399.
Now, evaluating Heyward's defense isn't quite that simple. Maybe the Braves throw a lot of fly balls (not really; they're 12th in fly ball percentage) or have an unusual number of starts made by right-handed pitchers, thus facing more lefties who hit the ball to right field (not really; the Braves are 20th in games started by right-handers). So there doesn't appear to be any team quirk that has allowed Heyward to make a high number of plays. He just makes a high number of plays.
In digging deeper into the DRS numbers from Baseball Info Solutions, we see Heyward also makes few mistakes. He has just one error and his total of 15 Good Fielding Plays - Defensive Miscues & Errors is +15, second only to Nick Markakis' +16 among right fielders. Heyward's arm has saved two runs -- nothing special there, although not a liability. It's all about running down fly balls.
Should we believe the numbers? The metrics agree on Heyward's performance on defense in 2014. Maybe you don't think one Heyward has been one of the most valuable players in the National League but I'm inclined to believe he has been.
(Although Kershaw would get my vote right now.)