I think both Casey and Nemec stay up until Luke is ready to return.
If Pesce is ready to go this week, I think Nemec comes out of the lineup and serves as a healthy scratch for a few games, maybe 2-3 games in a row. I dont think it will hurt Nemec too much development as he will be practising with the NHL team. Next week I would take Casey out for 2-3 games and insert Nemec back in so he doesn't lose too much in game conditioning. I would repeat the weekly process until Luke is ready to return. Of course if one of the other dman is banged up or Kovacevic game falls off the cliff, you can insert one of them as the team needs.
When Luke is ready to return send Casey and Nemec to Utica and call up DeSimone to sit regularly at the 7th dman, while Casey and Nemec play big minutes in Utica.