There's an underlying premise that the rotation on the Reds needs fixing.
The team may not feel that way. They have Cueto, Latos, Arroyo, Bailey, and Leake. Cingrani and Corcino will be knocking on the door very soon. They can add an emergency back up guy.
The team may feel with with the loss of Madson and Broxton, with Masset hurt, with Bray gone, that the safest course is to keep Chapman in the bullpen. It's certainly the easiest course.
My guess is Walt will play it both ways, look for starters and relievers, and make his judgment on Chapman depending on who else he can pick up.