I used Baseballreference.com. He was in positive territory this year, but not by much (defensively). Watching him play, he is average in my own estimation, at best. Obviously he has a cannon for an arm, and that is his best weapon defensively. I see him getting poor reads on balls all the time. I do not think he is much more than average defensively, or maybe slightly above ave. Also important: he refuses to play in RF where his best weapon, his arm, would be better used. That reflects a bad attitude. I think players should play wherever they are told to play and give it their best.
Offensively, in 130 PAs at Fenway Park (admittedly a SSS), his OPS/tOPS is .672/74. My guess is that the dimensions and layout of the park do not favor RHH line drive hitters where the wall blocks his hard hit line drives. Its also possible that its just not enough ABs. Everyone is aware of his alarmingly low OBP. Assuming he doesn't get traded we will see what those numbers are after 250-300 ABs. Its a contract year; they should go up.
Cespedes is not a bad player; he is a good player, but not an excellent one. I do not believe he is worth a long term expensive investment. If we can trade him and get value in return we ought to do it. That would help relieve the OF glut as well.