United emerged with a victory at Lincoln Financial Field but laboured in their second pre-season friendly after beating Scottish team Celtic 3-1 last week in Toronto.
The winning goal came in the 76th minute after Danny Welbeck intercepted a sloppy pass in midfield and ran straight through the defence before passing to Gabriel Obertan who slotted the ball home.
Bulgarian striker Dimitar Berbatov nearly extended United's lead, hitting the crossbar from close range in the 80th minute.
United coach Sir Alex Ferguson praised Philadelphia's aggressive play.
"It was the sort of game we needed," Ferguson told reporters after the match. "They were very good opponents."
The Scot declined to comment on speculation that central defender Nemanja Vidic, resting after his World Cup exploits with Serbia, might leave United, saying, "I can't clarify it at all."
Asked if he would play for the Manchester team this season, Ferguson replied, "I believe so."
Five minutes later, Union's in-form French striker Sebastien Le Toux found himself through on goal but dragged his effort wide.
The game's final chance came in injury time to United's Ritchie De Laet, who tried to chip the keeper but saw his effort saved.
