To an extent. IMO, if the Sox win a World Series, then the trade is worth it, even if Espinoza goes on to be a perennial Cy Young winner. Winning the World Series is the ultimate goal.
If Pom helps lead us to the postseason (which he is doing), but we fall short of a championship, then whether is was worth it or not becomes a little more shady. Might we have subsequently won a championship with Espinoza if he wasn't traded? Might we have eventually traded him for another player and subsequently won a championship?
There's no way of knowing for sure, but I can't go as far as saying that the trade was worth it just because we make it to the postseason. And again, I thought the trade was necessary, but so far, we've fallen short of the goal.