The Astros said outfielder Colby Rasmus underwent surgery Wednesday to remove a cyst from his ear.
Rasmus went on the disabled list Sunday after being bothered by the ear for some time. He is expected to return in September.
Hernandez is making his major league debut in center field against the Blue Jays and left-hander Francisco Liriano on Friday night.
Gregerson is 3-1 with 14 saves and a 3.09 ERA. He has been out since July 27 with a strained left oblique.
Hernandez hit .307 with 10 home runs and 53 RBIs in 107 games between Double-A and Triple-A this season.