Exactly 
Doolittle is turning into a disappointment. Unless there's an injury that we don't know about? Why would a top tier closer on a playoff team want to take a back seat in the bullpen?
Arm fatigue from overuse. Whether you want to consider that an injury or not. Hopefully he'll be over it soon, but meanwhile he's taking a back seat because he's smart enough and honest enough to realize that he wouldn't be doing anyone any good (him or the team) by pretending he's ok.
