McCullers was diagnosed with a muscle strain in his right forearm Monday, Julia Morales of AT&T SportsNet Southwest reports.
McCullers was pulled from Saturday night's start in the fifth inning after appearing to suffer an elbow injury. Although he'll be held out until September, the diagnosis is relatively good news for the Astros, as surgery isn't believed to be a necessary step in his recovery process. Houston will need to add a fifth pitcher to the starting rotation by Aug. 21. Brad Peacock or Collin McHugh would be prime options to slot in if Houston wants to pick from someone on the active roster. Otherwise there are a handful of deserving starters at Triple-A, although none of the top candidates are currently on the 40-man roster.