Muller was optioned to Triple-A Gwinnett on Thursday, David O'Brien of The Athletic reports.
Muller allowed six earned runs on five hits and three walks while striking out just one across 2.1 innings Thursday against the Reds. He was also charged with the loss, dropping his record to 2-4. Though Muller had pitched effectively in the rotation since early July, he is seemingly the odd-man out with Huascar Ynoa (hand) set to return Tuesday. Muller's demotion also suggests that Touki Toussaint will remain a starter until Ian Anderson (shoulder) is activated.