It seems to be the consensus that J.D. Martinez is the answer to the offensive woes of the 2017 Red Sox. And since he's not going to sign to be a DH, most seem to posit trading JBJ as the logical move. My questions are how much of an upgrade would he be, and at what cost is that worth it? Full disclosure: I generally am in favor of holding on to our homegrown guys. For one, that whole "devil you know" argument, especially in the wake of our big ticket FA deals over the last decade. More importantly, though, I like to follow these guys on their journeys, and after rooting for them for years, they're one of "our guys." It's not just laundry. And in this case, as frustrating as JBJ's streakiness at the plate can be, he's always a joy to watch in CF. He also seems to be a down to earth guy who has succeeded without any apparent freakish physical gifts.
WAR is the average of fWAR and bWAR. WAR/150 G is just for ease of comparison. Durability would obviously a factor.
JBJ
2014: 127 G, .531 OPS, 46 wRC+, 0.6 WAR, 0.7 WAR/150
2015: 74 G, .832 OPS, 123 wRC+, 2.35 WAR, 4.8 WAR/150
2016: 156 G, .835 OPS, 119 wRC+, 5.15 WAR, 5.0 WAR/150
2017: 133 G, .726 OPS, 90 wRC+, 2.9 WAR, 3.3 WAR/150
JDM
2013: 86 G, .650 OPS, 75 wRC+, -1.25 WAR, -2.2 WAR/150
2014: 123 G, .912 OPS, 154 wRC+, 4.1 WAR, 5.0 WAR/150
2015: 158 G, .879 OPS, 136 wRC+, 5.0 WAR, 4.7 WAR/150
2016: 120 G, .908 OPS, 141 wRC+, 1.8 WAR, 2.3 WAR/150
2017: 119 G, 1.066 OPS, 166 wRC+, 4.0 WAR, 5.0 WAR/150
Both guys struggled through a period of adjustment at the plate as they started their MLB careers, JBJ's being more severe as we all know. I included the season before each figured it out. If we throw that out and look at the averages since they blossomed:
JBJ:
121 G, .793 OPS, 109 wRC+, 3.5 WAR, 4.3 WAR/150
JDM:
130 G, .936 OPS, 148 wRC+, 3.7 WAR, 4.3 WAR/150
Martinez is clearly the superior hitter, while Bradley the much better fielder at a position higher up the defensive spectrum. No surprises there. And, really, the difference in defensive value would be the sum of (Bradley-Benintendi in CF) + (Benintendi-Martinez in LF). It would still be significant. However, in rough overall value, the two are pretty damn close. Now, if you ascribe to the "no premium power hitter in the lineup makes everyone else worse" school of thought, then Martinez's skill set holds more value to this team as currently constructed. I don't know that the evidence for it is anything more than anecdotal, though. (Is it?) Two more very large unknowns: 1) What would JBJ fetch in a trade? and 2) What are their contracts going to look like? From my point of view, I don't see Martinez being worth it on a long-term contract in his 30s, making $20M+/yr unless trading JBJ significantly upgrades another aspect of the team and the team doesn't care about the luxury tax for the next couple years.