Rusney Castillo still a mystery for Red Sox

Rusney Castillo is now in the majors for the second time with the Red Sox, and despite occasional flashes of brilliance, we still don't know who he is as a player.

We do know that the Red Sox thought highly enough of his ability to sign Castillo to a seven-year, $72.5 million deal last August, and that he impressed during a 10-game stint with the club at the end of last season. But there remains a great deal of the unknown about Castillo, and the nagging injuries that have affected his playing time dating back to 2014 haven't helped.

After the Red Sox finally called up the 27-year-old outfielder in late May, Castillo hasn't exactly had the impact at the plate that many were hoping for. Through 15 games in 2015, Castillo has batted .245/.260/.306 with just one extra-base knock—the home run he hit to spark Sunday's eighth-inning comeback against the A's.

That Castillo is playing in competitive games on a regular basis for the first time in a couple years makes the task of analyzing his play all the more problematic. That he has fewer than 100 career plate appearances at the MLB level only adds to the difficulty in determining just what kind of player the Red Sox have on their hands.

The one trend that does stand out with regards to Castillo is where he's hit the ball so far in his big league career. As the spray chart below demonstrates, the Cuban native has hit loads of groundballs to the left side of the infield, a tendency he showed last season and one that's continued into 2015:

Source: FanGraphs

Through Tuesday, Castillo has hit an astounding 68.4% of his balls in play on the ground. That rate ranks third highest in all of baseball among hitters with at least 50 plate appearances and first in the American League. Conversely, his 7.9% line-drive rate is the fourth lowest of any batter in MLB, which certainly doesn't bode well.

Still, this is where the unknowns surrounding Castillo come into play. Are these results largely a product of Castillo working off the rust from sitting out of games for the better part of two years? Or are these numbers an early sign that he is going to struggle against big league pitching?

During his 10-game stint with the Red Sox last season, Castillo began to show that he was improving at the plate. In his final four games, he went 8-for-15 with three extra-base hits, including two home runs.

Perhaps, then, Castillo will work through his groundball issues if given more time? It is hard, after all, to glean many meaningful conclusions out of such a small sample of plate appearances.

The manner in which opposing hurlers are attacking him lends some greater insight, though Castillo isn't exactly being pitched in a way that is abnormal or surprising. He's seeing fastballs well over 50% of the time, according to Brooks Baseball, but pitchers are also throwing him plenty of breaking balls when ahead in the count.

What does stand out is where pitchers have been targeting Castillo, with the majority of fastballs the outfielder sees being thrown right in on his hands:

This isn't atypical, but Castillo didn't see nearly as many inside fastballs last season, and it's worth wondering if opponents are banking on the notion that Castillo can't drive good heat on the inner half.

That pitchers have complemented these inside fastballs with low-and-away breaking balls isn't exactly telling either. After all, the idea of attacking hitters with a "hard in, soft away" approach is pretty standard for pitchers, and Castillo certainly won't be the first batter to see his fair share of breaking balls on the outer half before he proves he can handle them.

Nevertheless, Castillo hasn't been able to adjust to this approach yet this season. He's walked just once in 50 plate appearances (even though his overall swing percentage has dropped six percent), and he's making less contact both inside and outside the strike zone.

The need for adjustments shouldn't come as a surprise for a player with such little experience in the majors, and the fact Castillo drove the ball over the monster Sunday is encouraging.

It's also worth remembering that the Red Sox didn't sign the Cuban native solely for his bat. Castillo's defense and speed are his best attributes, and the early returns on both have been positive. While drawing conclusions about his defense over a 25-game period is unwise, the advanced metrics have rated him highly; and just going by the eye test, Castillo certainly appears to have above-average range in the outfield.

The next 25 games will tell us more about Rusney Castillo, and the questions that surround just who he is as a player will hopefully begin to grow clearer. That Castillo potentially has more to give the Red Sox on offense is indicative of the rest of the team as a whole. Boston's bats have underperformed up until now, but Castillo is just one of many hitters who should improve in the weeks ahead.

For now, however, Castillo is someone whose true talent is hard to measure, and the Red Sox will have to hope he can begin driving the ball and adding offensive value to his game soon.