Late in the 2014 season, the Red Sox wrote a check worth $72.5 million and handed it to a stranger. The organization had barely missed out on prized Cuban first baseman Jose Abreu less than a year earlier, and they ensured they wouldn’t be topped in the international market again by giving Rusney Castillo the largest-ever deal to a Cuban defector. Castillo wasn’t a complete stranger, of course. They’d seen him play plenty of baseball, they’d met him in person; he was no more a stranger than any other international free agent coming to America. But with Abreu, there was the kind of raw power that can’t be ignored. With guys like Yoenis Cespedes and especially Yasiel Puig, the tools were off the charts. Undeniable freak athleticism.

With Castillo, the figures of the contract could be difficult to see through a thick layer of foggy uncertainty. The upside and athleticism were there, of course. Even the Red Sox don’t hand out seven years and $72.5 million without upside and athleticism. But there wasn’t the Abreu power. There weren’t the Cespedes and Puig tools. Instead, there were concerns of a swing deficiency, and “fourth outfielder” labels, and comparisons to Rajai Davis and Shane Victorino. It was uncertain what the Red Sox new Cuban investment might be, as is the case for any Cuban investment. It’s just that, with Castillo, the comps weren’t as rosy, especially in contrast of the instant success stories of Abreu, Cespedes, and Puig.

Which brings us to the present. Here we are, more than a year later, with Castillo having played in parts of two seasons, and the thick fog of uncertainty still looms, and now it’s shading our view of the Green Monster, locked into a place in left field on Opening Day for a team with hopes of contention and little in the way of a viable, everyday backup plan in the event that the fog happens to grow thicker, and darker.

What can we make of Rusney Castillo? What’s gone right? What’s gone wrong? Where are the points of optimism, pessimism?

Let’s begin with a positive. In 703 career innings in the outfield, spread pretty evenly across the three positions, Defensive Runs Saved pegs Castillo as a +17 defender. Ultimate Zone Rating says +12. Baseball Prospectus’ Fielding Runs Above Average has +9. These type of numbers are always meant to be regressed, especially in a limited sample like the one we have. But when they all agree, and they all agree on such an extreme figure, they’re trying to tell you something. And that something is: based on what we’ve seen so far, there’s little reason to be believe Castillo can’t be a truly elite defensive outfielder, and he just might be one already.

The range has graded well. The arm has graded very well. And, while Castillo is relatively unfamiliar with playing left field, and especially playing left field with the Green Monster behind him, early reports seem to indicate that he’s adjusting to his home park’s quirk with ease. Castillo looks to be an asset in the field, and the one thing every scout seemed to agree on was that Castillo has plus-plus speed, even if he doesn’t have great base stealing instincts. The thing about plus defenders with speed is that they don’t need to hit much to be valuable players, which is particularly relevant to this conversation, because Castillo is coming off a season in which he didn’t hit at all.

Castillo’s slash line through 329 major league plate appearances is .262/.302/.379, which is laughably similar to the .268/.308/.430 line that lead prospect analyst Dan Farnsworth predicted when he extensively analyzed Castillo’s swing last year. Excerpts from Farnsworth’s piece include “shoulders stay very level” and “bat flat as it comes into the zone” and “little natural lift in his swing” and “swing path is going to be the limiting factor in Castillo’s ultimate ceiling.”

To wit:

It’s not all bad; Farnsworth finds some positives in Castillo’s swing, but, from the get-go, there were scouting concerns with regards to Castillo’s swing plane, and now here’s a leaderboard of the highest rookie ground ball rates we have on record. The data stretches back 16 years, and I set the minimum plate appearance threshold at 200, because that’s when ground ball rates stabilize. In a sample of 571 rookie seasons, Castillo just posted the sixth-highest ground ball rate. Nearly two-thirds of his balls in play were on the ground. If you clicked the link to the leaderboard, you saw Castillo’s company, and if you’re a Red Sox fan, you cringed. Of course, there’s plenty more to a batter’s success than ground ball rate, but it’s been the most extreme part of Castillo’s game, and it mirrors the scouting concerns from before he’d ever stepped foot on a big league field.

I wanted to look at something. I wanted to look at some of Castillo’s company, and what his ground ball rate peers did after their rookie season. Did any go on to be productive hitters? Were any able to add lift to their swing, and tap into any power? Castillo’s ground ball rate was 2.8 standard deviations above the rookie mean. Setting the minimum at 2 standard deviations netted me a group of 15 ground ball rate comps:

Castillo, of course, is a very unique case. He’s a player with a far higher prospect status than most any player on this list, and is regularly facing American pitching for the first time in his life. To compare Castillo to this group based solely on ground ball rate is unfair. At the same time, when Castillo signed, we heard talk from scouts of 20 home run potential, and now it’s hard to ignore the ground balls, and this group. No player who entered the league hitting as many grounders as Castillo has ever developed much in the way of power, or even a productive skill set. It’s certainly not a death sentence, but it’s a cause for concern.

The name at the top, Yelich, is promising, but Yelich also walks twice as often as Castillo, and Castillo’s game has always been built on an aggressive approach. You’ve got to go a bit further down the rookie ground ball rate list to find another promising comp, but eventually you do see Alex Rios, who entered the league with a 57% ground ball rate as a 23-year-old, and four years later had dropped that by 20% while slugging 24 homers. It’s not that it can’t be done, it’s that it’s rare.

In August, there were reports of an adjustment by Castillo to alter his swing path. Just a couple weeks ago, more talk of an adjustment, to shorten the swing to better adjust to major league velocity. Castillo, Boston’s coaching staff, or both, understand that the current swing doesn’t seem to bode well for significant offensive growth.

Shortly after the first swing path adjustment article surfaced, Castillo hit an opposite field dinger, with this swing:

Look hard enough and you’ll notice some subtle changes; the hands start in a different place, the leg kick is a bit more pronounced, it’s got a bit more loft, and probably other things I don’t see because I’m speaking out of turn here. Point is: it’s a work in progress.

Bigger point is: in 90 career games, Castillo’s been worth 1.3 WAR as a pretty bad hitter. Given the tools, Castillo’s probably got a relatively high floor. Even if he goes the route of his rookie ground ball rate peers, the defense and speed keep Castillo an average player. An average left fielder might not be worth seven years and $72.5 million, but the money’s already spent at this point, so there’s no worth fretting. Now, the Red Sox just want to see how much they can squeeze out of Castillo. For the bat to turn Castillo into a real threat, there’s got to be some more power, or there’s got to be some more on-base. Something’s got to give, either with the swing, or with the approach. The work is being done. So far, it’s just been tough to see.