Rusney Castillo is Boston's starting left fielder. We've known this basically since the Red Sox signed him in 2014 to no small fanfare, and while 2015 was essentially a lost season, it didn't change down the stretch, as Castillo started some 45 games over the last two months out there.

Those games, however, did not go well, and neither has Castillo's spring if you're going to focus on the results rather than the simple fact that he's managed to stay on the field. It's enough to get people talking, and that includes, to some extent, John Farrell:

Farrell suggested Brock Holt would see a good number of ABs in LF. Starting there again tonight. Wouldn't stun me if he's there April 4. — Tim Britton (@TimBritton) March 25, 2016

The concept of a Brock Holt - Rusney Castillo competition is fascinating to me, because there are so many similarities between the two. This despite the fact that they entered the league in such different ways. Rusney was a much-hyped international free agent, Holt a fringe prospect included as a secondary piece in the Joel Hanrahan trade.

Since making the jump to Major League Baseball, however, they've both been fast converging on the same point, with each facing similar concerns. If the hopes for Castillo were once that he'd be a very good player, now Red Sox fans are just hoping he's viable. Where fans were once hoping Holt would be a utility player, now he's actually got an All-Star appearance under his belt.

The question for Holt, though, is why he keeps fading. In the first half, he's very much deserving of that All-Star nod, with a .309/.373/.430 line to go with some serious defensive value. After the break? .241/.294/.306. He just can't keep it going.

There's a number of ways to interpret that, of which two are the most convincing:

1. Since it's only been two seasons, we're just seeing patterns where they don't actually exist. Brock Holt's hot streaks have come early where they were just as likely to come late, and the player we see as a whole is who Brock Holt really is, simple as that.

2. Brock Holt lacks endurance, and wears down as the season drags on. This is what we saw in the early days of Kevin Youkilis' career. In 2006, he hit .297/.407/.467 in the first half, .258/.347/.381 in the second. In 2007, those numbers were .328/.419/.502 and .238/.356/.391. We all know what happened when he figured that out. Brock Holt won't be prime Kevin Youkilis, but if he can last the long haul and get a similar boost? That's a hell of a baseball player.

For Holt, the durability questions are actually something of a positive. If we knew for certain they didn't exist, we'd essentially be left with scenario one, which does little but drop his ceiling a long way.

For Castillo, the situation is similar, but not quite the same. He, too, has durability issues, but those durability issues have kept him off the field. And beyond that, the player he's been when he's made it into the lineup, whether because of the injuries or because of talent, is just not good enough. For Castillo, he needs those durability issues to be important--something that can be fixed--because otherwise he's just...not all that good.

The spring numbers bode ill for Castillo, but not really because they're spring numbers, and will be wiped away the second April 4th arrives. And that's when Castillo's chance to prove himself will come. Hopefully, it's not a half-chance, either. John Farrell has, at times, been reluctant to play Castillo regularly even when he's up on the major league roster. And this quote on Holt would suggest we might be in for more of the same in that department.

But the best Red Sox team does not feature Brock Holt in left field. It features a good Rusney Castillo in left and a good super-utility Brock Holt with as light a workload as possible. Whether or not that's actually possible isn't clear, but it won't be clear until Castillo gets a prolonged stretch of at bats while fully healthy. And if the time for that isn't now, then when? The first couple months of the season are going to involve a lot of discovery for Boston. The expectation is that they're going to hit enough of their coin flips that, when combined with their solid core (Ortiz, Pedroia, Betts, Bogaerts, Price, etc.) they're able to keep themselves in position to spend at the deadline to replace the pieces that haven't worked out (if there's no internal replacement) and push onward into October.

So let's hold off on any and all "Holt in left" fever for now. The time may come where that's Boston's best move, and it may come sooner rather than later. But the season is a marathon, not a sprint, and using resources wastefully in an attempt to maximize early returns is a good way to fall short at the end, and hurt your chances in the future. Brock Holt will still get plenty of at bats all over the field even while he's not starting, where for Rusney Castillo, there's really only one clear role to play. Let's give him one more chance to show us he can do it.