Hjelle was recalled from Triple-A Sacramento to serve as the 29th man for Thursday's doubleheader against the Brewers.
Hjelle made three relief appearances for the Giants earlier this season and gave up six runs while striking out nine in six innings. He's typically worked as a starter for Sacramento this year and could be available to make a spot start or serve as a multi-inning option out of the bullpen for the second game of Thursday's twin bill.