Muller was recalled from Triple-A Gwinnett to start Sunday's game at Cincinnati.
The left-hander served as the 27th man in Monday's doubleheader and allowed three runs over five innings during the first start of his MLB career, and he'll take the mound again in Sunday's series finale. Muller could receive a longer look in the rotation with another decent start, since the expected return of Max Fried (blister) from the injured list this week will still leave Atlanta without a fifth starter.