Muller gave up one hit over two scoreless innings in Sunday's Grapefruit League game against the Phillies, striking out two without walking a batter.
It was a good first appearance of the spring for the southpaw, who's competing with Tucker Davidson and Kyle Wright for a spot at the back of the Atlanta rotation. Muller, a second-round pick in the 2016 Draft, got his first taste of the majors last season, and while he has a strong arsenal of pitches, his 12.9 percent walk rate will need some work if he's going to find consistent success.