What I said was if Chapman fails, Broxton succeeds and then the Reds have to decide what to do with that situation. Sure a triple negative could occur as well, so could a double positive (Broxton and Chapman succeed)
On the topic of injuries, there are vast differences between the injury rates of athletes. This is something to keep in mind, not just for Chapman but for any athlete. I'll refrain from posting it though since it would be long. In short according to a few papers, there are differences in injury rate for not only a RP vs SP, but also pitcher vs position player, different sports, and even between practice and games. Lofton and Glavine made good decisions to go with baseball as it offers a lower injury risk, in fact the lowest injury risk of all Male collegiate sports.
Again I am not arguing to keep Chapman in the bullpen, I think they should use him more as he is potentially a very valuable player. His value is much less as a closer compared to a starter. But there are risks and injury is one to consider. That is all I want to get across. The risks are real, but like you said there is also a risk of losing out on the extra production you could get from Chapman as a starter.
The numbers posted by Nomo are a bit underwhelming, hopefully him finding himself last year will result in him moving forward as a starter.
Between 2011 and 2012 his control did improve. Zone% from 44.4% to 49%. He also had a higher swing% and lower contact%.
http://www.fangraphs.com/statss.aspx...latediscipline