People have been comparing Arraez to Lopez in arguing that that was a bad trade. As I see it, the trade is really about, "Have the Twins gained more by having 15 starts of Lopez compared to 15 starts of someone else (which would have been someone like three starts of Ober before he was called up, but then 12 starts of Dobnak/Rodriguez/Sanchez, etc.) or would they have gained more by having 300 plate appearances from Arraez at the expense of 300 at bats from others (some combination of Kirilloff, Solano, Gallo, Larnach, Buxton)?
With how Arraez has been hitting, it's probably accurate that the offense would have been better with 300 plate appearances of him compared to those guys. But I think the gain from Dobnak, et al., to Lopez is much more significant than the loss from Arraez to others.
All of which to say is that it's never as simple as comparing the players who were traded. Every player has a context. I'd love to have Arraez, but in the context of what the Twins also had, I think he was among the most tradeable chips to fill in a need.
(And also, of course, that we're talking about a bit less than a half season. I'm not a betting person, but I will bet anyone on TD a buck that Arraez doesn't finish the year with a .400 batting average or a .940 OPS.)