Muller was recalled from Triple-A Gwinnett on Saturday.
Muller's call-up was expected since he'll start the first game of Saturday's doubleheader against Miami. He's posted a 3.27 ERA, 122:27 K:BB and 1.10 WHIP in 99 innings over 17 starts in the minors this year, but it's not yet clear whether he'll have a chance to remain in Atlanta's rotation beyond Saturday's start. His status may hinge on the availability of Max Fried (concussion), who is currently on the 7-day injured list but could be ready to go when first eligible Monday.