I wouldn't drop Fields either, even though he has Crede blocking him at 3rd, Kouzmanoff has Marte AND Aaron Boone (I Know that isn't much) blocking him. Though he is probably worth picking up, not at the expense of Fields...
I would probably drop Longoria (LONG way away)
Martinez (I know nothing about him, so use your discretion)
or Arredondo (also know nothing)