Yoho might be the only one eligible and/or ready to be called up on 9/1, so that might be what they are saving him for.
Patrick was optioned on the 25th, so he's not eligible to be called up until 9/8 unless someone goes on the IL (unless there's an exception for expanded rosters on 9/1, which a Google search did not reveal). Carlos Rodriguez is in the same boat, optioned on the 24th so not eligible until 9/7 barring an injury.
Anderson was placed on the IL on 8/25, so he isn't eligible to come off until 9/8.
Gasser could come off the 60-day IL, but they said they want at least one more rehab start from him which IIRC is on Friday, 8/29, so he won't be ready on 9/1.
D.L. Hall is not likely to be ready until at least mid-September. Henderson is done for the rest of the regular season.
Zastryzny could be ready, he's on a rehab assignment. It's either he or Yoho on 9/1. That's it (barring someone else going on the IL).