Atlanta selected Hernandez's contract from Triple-A Gwinnett on Thursday.
With Martin Perez (back) headed to the injured list, Hernandez will rejoin Atlanta's bullpen to fill the open roster spot. The 31-year-old has allowed just three earned runs in 11 innings with the big club this year but owns a 4.64 ERA and 1.43 WHIP across 85.1 innings in the minors. Hurston Waldrep (elbow) was placed on the 60-day IL to clear space on the 40-man roster.