Loewen's contract was selected from Triple-A Reno on Saturday.
The Diamondbacks announced a series of moves Saturday, and both Loewen and fellow lefty Steve Hathaway will join the major league roster. In corresponding transactions, the team DFA'd Josh Collmenter and Mike Freeman, while Zac Curtis was optioned to Double-A. Loewen is yet to pitch in the majors this season, but he's amassed a 3.52 ERA across 38.1 innings at Triple-A.