I would assume the 5 they have: Vargas, Shields, Duff, 'Tura, Guthrie are who they want all season. But I wouldn't be shocked to see Ventura or Duffy skip a start at some point once or twice in July/August. Also I'd assume they set the rotation for those two to get lots of rest at the All Star break. They way it is currently neither of them would go very far past those limits anyway.
If they do skip a start I think Chen is what the organization would like to have fill in, but he's been poopy in Omaha so he's not ready.
Also this is all speculation on my part.