|Union GK Brian Sylvestre - courtesy of the Philadelphia Union|
The Union, making due with their fourth goalkeeper in what has been a disastrous season so far, controlled the action throughout and held possession for 55 percent of the match.
That possession was key as the Union defense did not allow D.C. United (6-2-3, 21 points) to put any shots on netminder Brian Sylvestre, who recorded a clean sheet a day after Philadelphia (2-7-3, 9 points) extended his short-term loan from the Carolina RailHawks for eight weeks.
This was the second outing for Sylvestre, 22, who made his MLS debut for the Union on May 9 against Vancouver Whitecaps FC and made seven saves, a season-high for the club.
Pfeffer scored the late winner off Fabinho's pass in the third minute of stoppage time securing just the Union's second victory of the 2015 MLS season.