Hernandez was placed on the 10-day disabled list Sunday with a left oblique strain, but the Phillies were uncertain at the time how long their leadoff hitter would be sidelined.
Hernandez, 27, suffered the injury while making a throw last week. He is batting .277 with five home runs and a team-leading 40 runs scored this season.
Hernandez's extended absence could prompt the last-place Phillies to promote second base prospect Scott Kingery, who is batting .306 with 18 home runs and 14 stolen bases for Double-A Reading.
Kingery, 23, was rated as the Phillies' No. 6 overall prospect earlier this year by ESPN's Keith Law.