I guess this comes down to whether DD thinks Chavis' breakout this year is real. If it IS real then that would only be defensible if the current injury concerns - Kelly and, more importantly, Carson Smith are serious.
If Chavis keeps this up for the whole year then you'd have to think he's a top 50 prospect - which is not something you want to peddle for a marginal upgrade like David Robertson.
There's nothing marginal about the upgrade. Joe Kelly is smoke and mirrors and is almost certain to regress heavily, Matt Barnes is not reliable enough to be a setup man (his poor road stats match up with the rest of his career really well, so I'm guessing his home numbers are a mirage) and there's really nothing else in the pen that's been all that good unless you want to put your faith in Fernando Abad. There are some pretty ERA's but when you look past them, the bullpen is fairly ugly behind Kimbrel.
David Robertson would immediately become the second best reliever on the team. As for Thornburg, I wouldn't count on him getting back on the mound again this season. Thoracic Outlet syndrome is no joke. And Carson Smith has already had to be sidelined once in his attempt to return from TJ surgery, so counting on him is also probably a bad idea. Plus, even if he gets back on the mound, control is usually the last thing to return while recovering and I'm not sure that's what I want to be dealing with late in games down the stretch or in the playoffs.
They need to add at least one reliever, even if they are high on Workman and Maddox.
And to respond to the idea that Chavis is a top 50 prospect, that's iffy but a defensible position. BA has him in the top 100 right now, so he's not far from that already. And even if you assume his Salem numbers are at least partly due to a hot streak where he was hitting beyond his actual talent level, given what that stadium does to power numbers, there HAS to be some level of sustainable power there to end up where he did. So yeah, I think he could be a top 50 guy going into next year. He's got 4 HR in Portland already, so the power is carrying over.
That said, where would he play in Boston? Devers is clearly ahead of him in the depth chart and is a much much better defender than Chavis, so he's not getting pushed over to 1st for him. He's only 5'10" so he's not really tall enough to play 1st regularly. Hanley is tied up for next year and probably 2019, so DH isn't open any time soon.
The best value he can provide to this club is likely as a trade chip, and even if the breakout is for real, he's not going to be the kind of prospect who can headline a blockbuster. So why not let him carry the bulk of the load in getting two pieces needed for a playoff run this year from a team that can afford to eat those contracts and not put the Sox over the LT threshold? And keep in mind that Robertson is under contract for next year, too. So there's some extra value there.
Given how thin the system has been and how much depth they were able to work in through the draft and the IFA signing period, I think I'd rather see one solid but not great prospect going to Chicago with a fringe guy added in than 2 or 3 solid prospects from the new influx of talent. The system needs depth right now, and Chavis would offer a more efficient path toward paying for Frazier and Robertson, should that be what Dombrowski is trying to do.