Free-agency checkup: Which stars are adding and losing money?

Let's check in on some members of the so-called greatest free-agent class ever -- although it's admittedly a little less great given the results and/or injuries of Bryce Harper, Clayton Kershaw(if he opts out) and Josh Donaldson -- and how they're trending as we close in on the halfway point of the season. It's a deep class of potential free agents, so we're not going to touch on everyone -- but these are some of the names who have seen their value change the most throughout the first half of 2018.

Bryce Harper

Trending down: After a sizzling start in which he hit .315 and homered eight times in his first 17 games, Harper has hit .183 with 11 home runs in 52 games entering Tuesday's action. His average exit velocity is actually up 0.3 mph from last season and his average launch angle is up slightly, but he's hitting just .164 on grounders and has had some bad luck on line drives. Throw in some below-average defensive metrics and his WAR is 0.5.

What's left to prove: Has there been a less consistent player than Harper? At his best, like he was in 2015 when he won the MVP award, he's capable of historic numbers: He put up 10.0 WAR that season. In 2016, his WAR was 1.5. His plummeting batting average this season isn't helping his reputation as a player who can't put back-to-back great seasons together, let alone the back-to-back-to-back-to-back-to-back you'd want from a $400 million player.

Predicting the market: If Harper ends up with mediocre numbers, that could potentially bring more teams into the bidding process. The emergence of Juan Soto as a potential middle-of-the-order threat also gives the Nationals another outfield option to go with Adam Eaton, Michael Taylor and Victor Robles if Harper doesn't return.

Five potential fits: Nationals, Phillies, Yankees, Giants, Rangers. Rangers outfielders are hitting a lackluster .230/.316/.387. They have money coming off the books in Adrian Beltre ($18 million) and Matt Moore ($9 million) and potentially Cole Hamels if he's traded. Plus, their payroll this year was already about $30 million less than in 2017.

Manny Machado

Trending up: Machado has kept his focus as the team around him disintegrates and he's putting up his best season at the plate with a .310/.377/.567 line, with the lowest strikeout rate and best walk rate of his career.

What's left to prove: His defensive metrics at shortstop have been terrible, with minus-15 defensive runs saved, worst in the majors. This could be a Miguel Cabrera-like situation, when the Tigers were willing to live with the glove at third base, but some teams might view Machado only as a third baseman.

Predicting the market: Machado will almost certainly get traded, and the biggest question is if the Orioles would be willing to trade him to the Yankees or Red Sox -- and demand Miguel Andujar or Rafael Devers in return. If one of those trades happens, that opens up a natural landing spot for Machado for 2019.

Five potential fits: Yankees, Red Sox, Phillies, White Sox, Rangers. We'll point to the Rangers again. Beltre can still hit, but he'll also be 40 years old in 2019. The Rangers need a cornerstone offensive player, since it's probably not going to be Joey Gallo, and Rougned Odor's deterioration has made him almost unplayable.

Josh Donaldson

Trending down: Ouch. Donaldson's injury-plagued season has cost him untold tens of millions. He has played just 36 games, hitting .234/.333/.423. Remember, he was 27 when he finally became a full-time player, so he'll hit free agency at 33.

What's left to prove: He's currently on the disabled list with a calf problem. He has to get healthy, and if he rakes for two months like he did from 2013 to 2017, teams can maybe write off his slow start to the injuries.

Predicting the market: Given the emergence of Vladimir Guerrero Jr., it seems likely he'll be the Blue Jays' third baseman in 2019 and they'll move on from Donaldson. Still, there should be a lot of interest in Donaldson as a discounted version of Machado. Given the performances of some recent 30-something free agents, Donaldson probably won't get the six- or seven-year deal he might have received if he'd remained healthy all season.

Five potential fits: Cardinals, Phillies, White Sox, Rangers, Angels. Given their franchise history in free agency, the Cardinals are unlikely to join the Machado fray, but Donaldson could help provide some stability to their infield. The Angels have received subpar production from Zack Cozart at third and Ian Kinsler at second. Kinsler is a free agent, so they could slide Cozart over to second in 2019 (in fact, the Angels are a possible landing spot for Donaldson in July).

Kelvin Herrera

Trending up: The Nationals just acquired the two-time All-Star to help set up Sean Doolittle. The right-hander doesn't pump it up to 100 mph anymore, but he still averages 96.4 mph with his fastball and he has added a slider to his repertoire the past couple of seasons.

What's left to prove: The strikeout rate is down from his peak levels, but he also has become a strike-throwing machine, with just two walks in 25 innings. Some evaluators might look at the decline in strikeout rate as a red flag, but the overall numbers remain impressive.

Predicting the market: Relievers were the one group that got paid last winter, and there's no reason to expect that philosophy to change. Herrera has been good, he has been healthy and he has pitched well in a lot of big moments in his career. Almost every team will probably inquire about him.

Five potential fits: Cardinals, Phillies, Indians, Mets, Braves. These five teams all rank 19th or lower in bullpen ERA. Greg Holland has been a disaster for the Cardinals, but at least it was just a one-year deal. The Phillies have had late-inning issues all season. The Indians are looking at the potential free-agent departures of Andrew Miller and Cody Allen.

Yasmani Grandal

Trending up: Grandal was supposed to share time with Austin Barnes (who had been the regular starter in the postseason last year), but he has played so well that he has started 49 of the Dodgers' 70 games. He's hitting .243/.332/.450 with 11 home runs.

What's left to prove: Grandal is a pretty reliable and predictable player at this point in his career. He has a little pop, will draw some walks and ranks first in the majors in StatCorner.com's framing metrics.

Predicting the market: Seven teams are hitting less than .200 at catcher. Half the league has an OPS less than .650. If we're not at an all-time low in catcher offense, we're close to it. Grandal will turn 30 in the offseason, but there will be a lot of interest.

Five potential fits: Dodgers, Diamondbacks, Twins, Red Sox, Nationals. The Red Sox and Nationals are obvious suitors, ranking 28th and 30th in the majors, respectively, in catcher OPS.

Eduardo Escobar

Trending up: Holy cow is this guy having a season. Escobar had played a lot in recent years as a super-utility guy for the Twins, but he has played every day this season after the suspension of Jorge Polanco and he has hit 31 doubles -- that's a pace of 73 two-baggers. The record is 67, set by Earl Webb way back in 1931.

What's left to prove: Escobar's power isn't a complete surprise; he hit 21 home runs in 457 at-bats in 2017. He's not only on pace for 73 doubles, but 108 extra-base hits? Insane. The franchise record for extra-base hits in a season is Tony Oliva's 84 in 1964. Escobar is better at third base than shortstop, but if he keeps mashing, he'll earn a full-time job somewhere in 2019.

Predicting the market: The third-base market was already going to be saturated with Machado, Donaldson and Beltre and probably Mike Moustakas, and now you can add Escobar to the mix.

Five potential fits: Phillies, Orioles, Blue Jays, Twins, Indians. Given Miguel Sano's uncertain future in Minnesota, re-signing Escobar might be a priority. The Jays could use Escobar at shortstop, unless they think Bo Bichette can remain there. But here's an idea: The Indians move Jose Ramirez to second base and go with Escobar at third -- adding another switch-hitter to go with Ramirez and Francisco Lindor.

Dallas Keuchel

Trending down: Keuchel has been fine with a 4.15 ERA, but given his fastball velocity and some minor injury issues in the past, he hasn't pitched well enough to receive a $100 million mega-deal.

What's left to prove: Nothing, really. He is who he is, a guy who gets ground balls and won't beat himself. In 2015 and 2017, when he had sub-3.00 ERAs, his ground-ball rates were over 60 percent. In 2016 and 2018, those rates have fallen below 58 percent. With Keuchel, it's a fine line between being a No. 1 or 2 starter and a No. 3.

Predicting the market: Keuchel and Charlie Morton are both free agents. The Astros are unlikely to sign both, as prospect Forrest Whitley should be ready in 2019, plus you still have Collin McHugh as a rotation option. Keuchel and Jose Altuve are the two Astros who suffered through the really bad days. They gave Altuve a big extension, but I'm not sure Keuchel receives the same love.

Five potential fits: Astros, Rangers, Padres, Angels, Yankees. Keuchel always seem to pitch well at Yankee Stadium (2.45 ERA, no home runs in 33 innings, and that doesn't include his wild-card start there in 2015).

J.A. Happ

Trending up: Happ has gone 38-18 with a 3.36 ERA under the three-year contract he signed with the Blue Jays before the 2016 season. He has a 3.48 mark this year and his 28.1 percent strikeout rate is not only a career high, but 5.4 percent higher than 2017.

What's left to prove: He's not a big workhorse, averaging fewer than six innings per game, and he'll be 36 in October, but the lefty has figured out how to succeed by throwing over 70 percent fastballs and sinkers. He's showing no sign of decrease in velocity, averaging 92-plus with his fastball. If he can stay healthy, he could get another three-year deal despite his age.

Predicting the market: He's prime trade bait right now, and the Yankees and Mariners already have inquired, according to reports. While the market for second-tier starters last winter -- guys like Lance Lynn and Alex Cobb -- was cold, Happ's strikeout rate would seem to put him in a more desirable category. Plus, he's proving he can succeed in the American League East.

Five potential fits: Yankees, Mariners, Rangers, A's, Mets. Hey, the Mets will need some pitching after they trade Jacob deGrom and Noah Syndergaard (I kid, Mets fans, I kid ... maybe).