The one thing that we know about the second half of the season is that it won't be identical to the first. We've learned a lot in the first half about who the good players and teams are, but we've also learned enough over the years to know that there's more than enough room for first-half underperformers to become second-half studs.
Of course, that only works if there's actually something positive under unimpressive stat lines. A player who hasn't hit in the first half because he strikes out too much and makes weak contact has probably earned every bit of it, and he shouldn't be expected to improve. On the other hand, sometimes you can identify players with underlying skills that ought to bring more production. For example, on May 24, we noted that Jackie Bradley Jr. was hitting too hard to not be more successful. At the time, he was hitting just .200/.277/.360; since then, he's hit .331/.417/.573.
So let's see if we can do that again, using Statcast™ data to identify underperforming hitters who just might be positioned to have a better second half.
Manny Machado, Orioles
Is it fair to say that Machado has been one of baseball's bigger disappointments in the first half? After becoming a true superstar in 2015-16, hitting a combined .290/.351/.518, he's hit just .230/.296/.445 this year -- or exactly the same OBP and slugging he had as a 19-year-old rookie in 2012.

Yet it's difficult to square that production with this simple fact: No one in baseball has hit more balls at an exit velocity of 95 mph or harder than Machado, who has 128. That's our "hard-hit" line, and for good reason: The Majors' average is .557 with a 1.141 slugging on balls hit that hard. Machado, though, is hitting "only" .468 with a 1.024 slugging on his hard-hit balls, meaning he's not getting the production he ought to on these high-value hits. (Last season, he hit .608/1.251 on them.)
So Machado has still got his hard-hitting skills, and it's really hard to mash that much without production following. It's possible we've already seen the turnaround begin. In July's first nine games, he hit .333/.350/.615 with three homers and only five strikeouts.
Matt Carpenter, Cardinals
Carpenter's case is a little different, because unlike Machado, who has been below average, Carpenter hasn't been bad -- not really. He just hasn't been himself, because after a 2015-16 line of .271/.372/.505, he's at .237/.378/.449 this season, and you can see the issue. Carpenter's on-base percentage is right in line with where it always is, but he's missing over 50 points of slugging. Has he lost his power?

We have a metric called Expected wOBA, which sounds complicated, but isn't, really. wOBA is just like OBP, except it gives more credit for extra-base hits, rather than treating each time on base equally. The expected part of it is that we look at quality of contact via exit velocity and launch angle to see what a hitter earned with his contact skill, which is a good way to give someone credit for crushing a ball even if an outfielder turned it into an out with a highlight catch, since that has nothing to do with the hitter. (We also roll in real-world whiffs and walks to give a full picture.)
Of 346 players with 100 plate appearances, Carpenter's xwOBA ranks 12th, ahead of stars like Corey Seager, Bryce Harper and George Springer. But the gap between what may have been expected to happen and what did happen is higher than all but six other players, which is echoed in his unusually low .256 BABIP, since he's usually in the .320 range. It doesn't guarantee an improvement, but it does tell you Carpenter's famous bat speed is still intact. It's a good sign for a player nearing his 32nd birthday. 
Kyle Schwarber, Cubs
It's fair to say that Schwarber was never, ever going to live up to the expectations placed upon him by his miraculous World Series heroics last year, simply because no player could. No matter what he does from here on out, he'll always be part of the legend of the Cubs team that finally broke the curse, and that's a lot to live up to. (It's why we suggested the Cubs trade Schwarber at the peak of his value for badly needed starting pitching last offseason.)

Still, even if you believe Schwarber's ceiling is well below that of a superstar, it's also a near-certainty that he's better than the mere .171/.295/.378 line he put up before his brief demotion to Triple-A. That's partially because every scouting report, projection and pre-2017 stat line insists it's true; because he had the lowest BABIP of any qualified player in the first half, and because some of our more advanced Statcast™ metrics show he deserved more success.
Just like with Carpenter, the way Schwarber hits the ball shows that he probably deserves better (though his limited foot speed will always prevent some extra bases). Just based entirely on the quality of contact, and the amount of contact, he makes, Schwarber's expected outcomes are more like Andrew Benintendi and Mike Moustakas than the Freddy Galvis / Aledmys Diaz range he's in. It's not star-level, it's just better.
Jose Cabrera, Tigers
Have we really already counted out one of the best hitters to ever play? Cabrera's status is such that when he hits "only" .264/.357/.440, it's seen as a tremendous downturn, and while it is on track to be his worst line since he was 20, it's also still as good as guys like Dustin Pedroia and Robinson Cano. Still, Cabrera is 34, and he has dealt with a groin strain this season, so you could credibly wonder if he's beginning to decline.

Perhaps so, but like Carpenter, the bat speed still appears to be real. Cabrera's eighth on our list of top xwOBA, and absolutely no one in baseball has had more balls with an expected average of .500 or better turn into outs -- it's happened to him 32 times.
Now, as we've said often in the past, a big part of that is the vast outfield at Cabrera's home park, where you can do things like hit the ball 405 feet for an out, as had happened to him twice this year. But -- and we can't stress this enough -- Cabrera has the second-best hard-hit percentage of any hitter with at least 100 at-bats, behind only a hitter you may have heard is having a good year, Aaron Judge. Cabrera will be fine. Cabrera is fine.