My guess is that it's based purely on his performance the previous season, but my only bases for that is my assumption that MLB will calculate that sort of stuff in an exteremly arbitrary way.
ETA: I also assume that it's based purely on stats like BA, RBI, R, and FLD% rather than ones that give a more accurate picture of a player's performance.
"I don't buy everything I read,
I havn't even read everything I've bought"
"I find it more comforting to believe that this [life] isn't simply a test."