With Donn Roach getting the start for the Iowa Cubs today, it would appear likely that Dallas Beeler will get the call-up to be the "26th man" and start one of the games of the day-night doubleheader for the Cubs versus the Cardinals on Tuesday.
Beeler last started for Iowa against Round Rock in Des Moines last Tuesday and Roach last pitched a week ago yesterday for the Cubs in St. Louis, so unless the Cubs intend to start Edwin Jackson or Travis Wood on Tuesday, the choice was probably between Beeler and Roach.
Here is the how the "26th Man" rule works.