Jonny Gomes and Daniel Nava are both useful baseball players. Last year, the pair combined to make Boston's one of the more productive left fields in the league despite some damning defensive metrics that likely do not give them full credit for their ability to play that wall in left.
That being said, each of them plays a very specific niche, and outside of said niche, things can get very ugly indeed. Let's take Daniel Nava, put him in right field, and add a left-handed starter to the mix. Suddenly the .366 wOBA that Nava brought to the table drops to a dreadful .288. And that -15 UZR/150 starts to actually show through with regularity now that Nava is out of the cozy confines of left.
Much the same would be true for Gomes against a right-handed pitcher. His offense is likely to be better, but still not good, while his defensive abilities are even less suited to right than Nava's, particularly given his less conservative style and Fenway's tendency to turn defensive mistakes out there into inside-the-park homers.
Let's put both those bad sides together to make a player with a .305 wOBA and poor defense playing a full-time role. The closest I can get for representation of that over the course of the 2013 season is Nick Markakis, who hit .271/.329/.356 and was credited with costing his team about six runs in the outfield. Fangraphs and Baseball-reference agree that Markakis was a fine example of a replacement-level player.
Why do I mention all this? Because these days the idea of a Gomes - Victorino - Nava outfield is getting plenty of play. Not as a good option, mind, but potentially as a "necessary evil" in order to keep Grady Sizemore healthy without having Jackie Bradley Jr. as a backup. This, of course, is in the event that the Red Sox send Bradley down to the minors in order to keep Mike Carp on the roster come Opening Day.
The Nava - Gomes combination is the real cost of such a scenario, because in any game where both players start, one of them is going to be that replacement level player, effectively*. As such, when making any roster move, the Red Sox have to ask themselves: "how many games will this leave Nava and Gomes starting together, and how much will it hurt?" This is the Nava - Gomes test.
Assuming that Shane Victorino doesn't suffer unduly from making a right-to-center switch, we can consider this cost to be, basically, equal to what we expect the man who would otherwise be playing center field to produce (since we're subtracting a replacement-level zero). In this case, since Grady Sizemore would be making up for the remaining games either way, we can just default to the production of Jackie Bradley Jr. For that, we have only projections, which generally expect him to be worth 1.5-to-3 WAR.
What, then, is the cost of letting this replacement level player into the lineup? We need exact playing time numbers to determine that, which are hard to figure, but let's imagine that Sizemore plays two-in-three games. That doesn't seem too unrealistic in either direction. Given a one-win Bradley, that would make the drop-off pretty insignificant. Given a three-win Bradley, that would project to a full win in the standings.
Photo Credit: Kim Klement
Taken by itself, that figure--for the switch from a Nava - Bradley - Victorino combination to Gomes - Victorino - Nava for 54 games--might or might not justify giving up Mike Carp, depending on who's making the decision. It doesn't quite stand alone, though. There are factors both mitigating and aggravating.
On the one hand, this scenario assumes that Shane Victorino plays 162 games in 2014. He won't. Each day off for Shane Victorino will mean another day of Nava and Gomes playing together. If Victorino plays in 130 games, then suddenly we're looking at better than half-again that value lost to keeping Mike Carp. There's also the question of what it does to Bradley's development to lock him up in Triple-A again when it seemed like the team was ready to finally give him the keys in 2014.
On the other hand, disabled list trips will cut this number down again. If we accept that whoever ends up injured would have been injured anyways, then every game in that period will have the same players available in the outfield regardless of whether Mike Carp was kept aboard or not, since Bradley would come up to fill the spot if he was not already there.
Unfortunately, Mike Carp himself doesn't actually factor all that much into the equation, at least in 2014. Not if Daniel Nava has progressed at all in his ability to play first base. In the event that the Red Sox want to go lefty-heavy and give Mike Napoli a day off against a right-handed pitcher, Nava can shift to first with the Sox going defense-heavy in a Sizemore - Bradley - Victorino outfield. His role as a pinch-hitter is also more-or-less offset by Bradley's role as a late-inning replacement.
There shouldn't be much question that the Red Sox are a better-built baseball team with Jackie Bradley Jr. on the roster than Mike Carp. At least in terms of winning as constructed, he provides flexibility that will give the team the best chance to keep their outfield both healthy and productive.
The Red Sox, however, are not a short-sighted organization, and it does have to be taken into account the risk and reward. No matter what, it's hard to imagine the reward of starting with Bradley is greater than three wins (three being a significant number, mind), and not too hard to argue that it's worth less than a full win.
How much is keeping Carp worth? It's hard to say. He's kind of a mystery given his roller-coaster career. If in two years the Red Sox are turning to him to act as a big bat in a starting role, he could be quite valuable indeed. If in four months' time his trade value has crashed, then the Red Sox will have shot themselves in the foot twice over.
It's up to each of us to come to our own conclusions on how much Bradley is going to be worth and how much of a hit the team should be willing to take in order to keep Carp. Honestly, actually looking at these numbers, I'm less convinced than I was before that a Nava - Gomes outfield would come at an unacceptable cost. But it's certainly clear that the possibility is there for the damage to be significant.
*Carp himself can try to bear some of that burden, but his ability to play the outfield is rudimentary even relative to Nava and Gomes. He is not a realistic answer.