More intelligent posters than myself have argued that Espinosa's projected offensive value is much higher at SS rather than at 2B. (If I'm remembering correctly)
My problem with that argument is what if Desmond can't learn second base? Espinosa has already shown he can, and Desmond's range and arm show he has the talent to play short. I couldn't find a game log for fielding, but I am pretty sure he had more errors in less games before the all-star break. Even with all the errors Desmond still has an average major league player, and that was this teams main problem. They had replacement level players at center, right, catcher, and second for most of the year. Fill those four spots with average major league talent and you are looking at a 8-12 game improvement. Of course that also means you need to keep Adam Dunn or find anoth 4-5 WAR player for first.