The author here feels that the Castro call-up isn't much of an improvement over Theriot and Baker/Fontenot unless he exceeds expectations. This is purely a sabermetric view:
However this is portrayed by people, the Cubs did not call up Castro on a whim. Whether the thinking behind the move is misguided will be a consistent point of discussion during Castro’s tenure on the north side, and it will begin with the wins and losses this team sees as a result of their middle infield play this season. If Castro doesn’t have a .330 wOBA and +5 UZR/150 defense, then I really can’t justify what Jim Hendry is trying to do here. As I said, the benchmark has been set.