It's called a rotation. Fields played to fill in for an injured starter. As soon as Casey starts messing with the regular rotation there will be all kinds of complaints about how he can't settle on anything and establish a rotation that the players can count on, and i will be among those complaining, rightly.
Salmons is much better when rested though, and he probably should be given some nights off down the stretch. He played a really nice game after the four game break.