Muller will join Atlanta's major-league camp this spring.
Muller has yet to reach Triple-A and is therefore unlikely to push for an Opening Day roster spot, but his debut could come at some point this year. In 22 starts for Double-A Mississippi last season, he recorded a strong 3.14 ERA and a 25.6 percent strikeout rate, though his 14.5 percent walk rate was worryingly high.
Muller was forced to leave Friday's game with Double-A Mississippi due to an unspecified injury.
The location and extent of Muller's injury are unclear at this time. He'll be considered day-to-day moving forward.
Muller tossed 5.1 scoreless innings for Double-A Mississippi on Tuesday, giving up three hits and five walks while striking out five.
The 21-year-old southpaw has, in many ways, had a very impressive campaign in the Southern League, striking out 110 batters while serving up only five homers in 100.1 innings en route to a 3.50 ERA. However, Muller has also issued 62 free passes, with Tuesday's start being the third time in his last eight outings he's walked five batters. His big fastball and strong off-speed arsenal have allowed him to get away with it against Double-A hitters, but he'll need to improve his control significantly if he's going to hold down a regular rotation job in the majors -- otherwise, like Sean Newcomb before him, a move to the bullpen could be in his future.
Muller allowed one run on five hits and two walks over 6.2 innings while striking out six for Double-A Mississippi on Wednesday.
The 23-year-old had some control issues to begin the year, not surprising for a 6-foot-6 left-hander still trying to refine his mechanics, and he issued at least four free passes in four of his first five starts. He's back to pounding the zone now, though -- Muller fired 71 of his 106 pitches for strikes against Montgomery on Wednesday, and he sports a 2.27 ERA and 40:28 K:BB through 39.2 innings. A projectable second-round pick out of a Texas high school in 2016, he's added plenty of velocity over the last two seasons and now touches 96-97 mph with his fastball, complementing it with three off-speed pitches. Atlanta has no need to rush him given all the other high-upside arm in its system, but Muller is on course to be a mid-rotation starter who could make his MLB debut as soon as 2020.