Sonny Gray got sick and spent last night in the hospital, so he couldn't pitch today as he was scheduled to. The hypothetical man in the A's bullpen is Drew Pomeranz, but he pitched last night, so he was in no position to make a start. Nobody else in the 'pen is a starter, even on a part-time basis. Thus, the A's had to call for help from the minors in the form of Chris Bassitt.
Thus on top of thus, the A's needed to make a 25-man move. One thing they could have done, as suggested by friend-of-the-blog @ThanksBilly on Twitter, was send down Kendall Graveman.