The Giants selected Lonsway's contract from Triple-A Sacramento on Saturday.
The Giants are down a couple of starters heading into Saturday's twin bill against Arizona, so Lonsway will come up from Triple-A to give San Francisco an arm capable of handling multiple innings as either a starter or long reliever. The 27-year-old owns a 6.00 ERA and 1.85 WHIP through 72 innings at Sacramento and will be making his MLB debut whenever he gets into a game. Spencer Bivens was optioned in a corresponding move.