Keep in mind also that down the stretch last year the backend of the pen included Broxton.
It was Marshall, Broxton, Chapman in the seventh, eighth and ninth. Take Chapman out of the pen, and two of these three are gone (unless Brox re-signs).
Hoover potentially could fill one of the roles, but he has never pitched a full major league season as a late innings reliever.
So, when focusing on Chapman as a starter, which would be a fine experiment, you have to account for those late innings.
Right now, if no Chapman, the Reds late innings are covered by Sean Marshall and potentially JJ Hoover. Seems kind of thin to me, and not where the team wants to be.