My Initial reaction to this trade (when a coworker told me in the bathroom that we got Pomeranz for "a pitcher named Andrews or something?") I was pretty upset. Then I came to this site, scoured the internet for a bit, and started to feel a little better about it, before finally settling in on something like, 'The Padres 'won' the trade, but I can see why DD made it and it probably wasn't a terrible move now, regardless of how it turns out.'
1.) It makes the Red Sox better in the short- to medium-term. However, the degree to which it improves the team is debatable. Looking at just ROS projections, it seems like Drew could add 1 to 2 wins over the second half, compared to Buchholz (or whoever you choose to have him replace in your mind). Then maybe another 1.5-2.5 wins in 2017/8, depending on how much you believe in the new Drew (as a real improvement and not luck/outlier year).
2.) The Sox are in a position where the marginal wins are worth a bit more to them, given the division and wild card standings as they are now, plus the offense and Ortiz.
3.) For all of the arguments about how a kid in low-A is essentially a lottery ticket, studies like this one
http://www.thepointofpittsburgh.com/mlb-prospect-surplus-values-2016-updated-edition/ suggest that this is false in context, and that a pitcher in the top-tier of prospect rankings (which already incorporate temporal distance from the major leagues) has a greater than 50/50 shot of making meaningful contributions in the big leagues. Espinoza, while raw and young, is not some long-shot unicorn, but in fact likely to have a positive WAR while under team control.
Basically, whether or not you like this trade depends on a few questions:
Is the Drew of the last 2 months the real performance level we should expect going forward?
What is the likelihood that Espinoza becomes at least as good as Drew (career numbers) or Drew (2016 version)?
Will good pitching be as hard to find in 2018-2024 as it is now?
Will the Sox be in a position to compete for titles during that time period?
In the end, this move probably costs us a few wins over time, and is very likely to be a loss in terms of surplus value over the controlled years of both players, but front-loading those wins when we KNOW that we are in a position to be competitive vs potentially spreading them out over years when we aren't make the risk acceptable. I'm not sure that I make this deal 6 times out of 10, but the amount of conflict I have over it probably means that it doesn't lean too strongly in either direction. It's rare to see this kind of 1-for-1 star for top prospect trade, and this is probably why.