Hernandez last pitched on July 31 against the Texas Rangers, during which the right-hander allowed four runs on six hits and three walks.
King Felix has already spent time on the disabled list in 2017, missing nearly two months with shoulder bursitis. Since coming back from that injury, the 31-year-old has posted a 4.02 ERA and 5.32 FIP over eight starts – struggling in his last two outings in particular.
In a corresponding move, the Mariners have recalled left-handed pitcher Marco Gonzales.
The 25-year-old starter has made seven career starts in the big leagues and has a 5.53 ERA and 5.72 FIP. In a pair of starts with the Mariners’ Triple-A affiliate, Gonzales has a 4.50 ERA and 3.76 FIP.