Herdling has joined Philly on-loan from Hoffenheim.
"He's a good player and is a really nice guy," Philly midfielder Amobi Okugo, who started alongside Herdling during the club's 2-1 loss to the San Jose Earthquakes. "He's really positive. You can tell by the words he says on the field and how he helped our team move forward."
Union coach John Hackworth told MLSsoccer.com: "If you're a good player - and Kai is certainly a very experienced and quality player - it doesn't take a guy long to figure it out.
"He's adapted very fast. He's a very technically gifted player, very composed, and I think he sees the game at a very high level."