Muller was recalled by Atlanta on Wednesday.
Muller was added to the 40-man roster in November, and he'll now be in line to make his major-league debut after Tucker Davidson (forearm) was placed on the 10-day injured list in a corresponding move. The southpaw made seven starts for Triple-A Gwinnett to begin the year and posted a 4.60 ERA and 1.51 WHIP in 31.1 innings.