If Espinoza was in AA, you wouldn't trade him. Right now, he's just to far away to even project anything. Pitchers flame out more often than hitters. The Sox screwed up the rotation so bad (picking up Buchholz's option, relying on Kelly) that they basically forced this trade.
If they moved Espinoza for a rental, it would have been a horrible move. They moved him for a solid #2 guy who just pitched in the AS game.