The Reds' 40-man roster is still showing 38 players on the Reds.com website. Even if they haven't updated it yet, this means there are still two open spots that will be filled either internally or externally.
The most obvious candidates that aren't on it are Dontrelle Willis and Dave Sappelt. Neither of which, however, are going to be added unless they're simultaneously called (Willis because he can't be optioned and Sappelt because they wouldn't use up an option this late in the year if they could avoid it).
I'd almost be surprised if Willis gets a call right now with Chapman going back up. Even if Sappelt is one of them, they're not likely to carry six outfielders, so Gomes or Lewis would have to be sent outright (Lewis could refuse and become a free agent) or Heisey would have to be optioned (which seems unlikely).
I guess my point is... it makes one wonder if they're not working on filling a spot or two from outside the organization. It very well could be that they're working on clearing a few people through option waivers before they send them down and then they'll add a couple guys to the 40-man roster and call them up. But piecing together the corresponding moves they'd have to make, I'm curious if they aren't trying to put something together with another team.