Hjelle was optioned to Triple-A Sacramento on Monday, Evan Webeck of The San Jose Mercury News reports.
Hjelle appeared in two games this spring, allowing one run on two hits while striking out four over three innings. The 24-year-old has yet to make his MLB debut, but he did start 10 games with Sacramento last season, struggling to a 5.74 ERA and 1.81 WHIP with 35 strikeouts and 29 walks over 53.1 innings.