Start him or DFA him.
Personally, I'd start him. I'm wholly unimpressed with Treinen. He has a big fastball, that's it. Breaking ball is painfully average. No change.
I think Treinen can be a good ML starter, but not seeing it yet. He had one walk that should have absolutely been a strikeout tonight, and one was an intentional walk, but he still needs more command. Detwiler has command issues too, but we've seen even with his offspeed pitches not working for him he can be a pretty good starter, which speaks to how good his fastball is. And his offspeed probably won't ever be consistent, but when it works he's amazing, and when not he's serviceable to good. Plus it would help his confidence which probably needs a good boost right now.
I'd flip him if we could, but I don't think we could get anything worth the potential at this point with the way he's pitched out of the pen right now. Put him in the #5, let him get back to 2012/early 2013 as he can, and get some decent return or live with him. At this point, he's probably not happy with the Nats and wouldn't stay here even if asked, so give him a good tryout for someone—anyone.
With the way the offense is hitting right now, giving 1/5 starts to Detwiler I don't believe will be the difference between winning or losing the division. Just let him do what he's used to for a while. Hopefully it works out for both him and us, but if not, I don't think we're losing too much, even though starting in the majors would be good learning experience for Treinen or any pitcher.