Hernandez will start for the NL-leading Dodgers on Friday in Milwaukee. He will fill in for Josh Beckett, who is scheduled for an MRI exam after feeling discomfort in his left hip.
Hernandez, who turns 34 on Aug. 30, is 6-8 with a 3.87 ERA in his first season with the Phillies, winning his last three decisions in impressive fashion. Philadelphia signed him to a one-year, $4.5 million deal last December.
"Roberto provides us with another big-league starting pitcher who has pitched well this year, particularly the last seven starts," Dodgers general manager Ned Colletti said. "He has postseason experience and helps fortify the rotation down the stretch."
Hernandez spent last year with Tampa Bay and his first seven major-league seasons with Cleveland, making the AL All-Star team in 2010.
Hernandez was known as Fausto Carmona for his first six big-league seasons. The Dominican Republic native missed most of the 2012 season after getting arrested on charges of falsifying his identity to obtain a visa.
Hernandez has been a starter for most of his career, but could fill multiple roles for the Dodgers. Los Angeles manager Don Mattingly was excited about adding another veteran pitcher to his staff.