Hernandez could need Tommy John surgery, which would end his season.
Hernandez is the second Diamondbacks pitcher to suffer the injury this month, following ace Patrick Corbin, who was also diagnosed with a UCL tear. Corbin is out for the year after undergoing surgery earlier this week.
"It's just a tough pill to swallow," Hernandez said, via MLB.com.
Hernandez has 63 holds since joining Arizona in 2011, which is tied for the fifth-highest among National League relievers. He struggled through most of the 2013 season and was sent down to Triple-A Reno before finishing the year strong after his September call-up. Hernandez went 5-6 with a 4.48 ERA for the Diamondbacks last season, with 15 holds and 66 strikeouts in 62 appearances.