Hernandez, 33, is 6-8 with a 3.87 ERA in 121 innings across 20 starts and three relief appearances this season. Philadelphia signed him to a one-year contract worth $4.5 million over the winter.
The Dodgers lost Paul Maholm for the rest of the season to a torn ACL recently, and both Josh Beckett and Dan Haren (last start notwithstanding) have struggled of late. They needed another starter for depth.
In most cases, players to be named later in August are players on the 40-man roster who either did not or have yet to clear trade waivers. That doesn't necessarily mean that is the case for this deal though.