The visitors will look to maintain their Western Conference playoff spot when they travel to PPL Park
The Rapids (7-5-5) will be the more rested of the two sides, coming off a 1-1 draw with the Columbus Crew on July 4. The Union, meanwhile, held off New England 2-0 in Open Cup play on Tuesday night.
Despite falling 2-1 to FC Dallas on July 4, Philadelphia (4-8-6) is still just two points off a playoff spot in an Eastern Conference dominated by parity.
The Union will be missing Maurice Edu through red card suspension, while the Rapids are still without midfielder Jose Mari due to injury.