To those throwing the "tired arm" thing out there like some kind of conspiracy theory, keep in mind that he was A) never once diagnosed with a "tired arm" and B) it didn't end his season.
You're essentially projecting an imaginary hindrance on him because you think he should stay the closer.
If Chapman didn't have the stamina or pitches to be a big league starter, the Reds' brass wouldn't have signed him to a 6 year contract, they wouldn't have had him working as a starter last spring, and they wouldn't be planning on moving him again this offseason. End of story.