The Athletics selected Muller's contract from Triple-A Las Vegas on Wednesday.
Muller was outrighted off the 40-man roster last month but has made his way back on after posting a 4.43 ERA and 12:5 K:BB over 20.1 innings at Las Vegas. Three of Muller's four appearances with Las Vegas were starts, and he went a combined 13 innings his last two times out. However, while he's stretched out to start, if needed, it's not clear what role the Athletics have planned for him.