Hackworth took over on an interim basis on Wednesday when the team and Nowak decided to cut ties. The Union cited a difference in philosophy as the reasoning behind the coaching change.
Philadelphia is looking for its first Major League Soccer victory since April 21. The Union haven't played a league match since being defeated 1-0 by Toronto FC on May 26.
D.C. United is headed in a different direction than the Union, capturing its last three matches to move to an Eastern Conference-leading 8-4-3 with 27 points.
The Union defeated D.C. United 1-0 on June 5 in the fourth round of the U.S. Open Cup at Maryland SoccerPlex.
For more Philadelphia Union coverage, visit the SB Nation blog Brotherly Game.