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.