Rookie Danny Mwanga scored a goal in stoppage time for the third straight game, but this time it was too late to prevent a Union loss.
The Union's two-game unbeaten streak ended with a 2-1 loss to the Chicago Fire on Saturday night in Bridgeport, Ill., leaving the Union in a tie with D.C. United for seventh place in MLS' Eastern Conference.
"In the last 3 weeks, [Mwanga's] been working very hard," said Union manager Peter Nowak. "He's capable of doing these things."
Chicago, in fourth place in the East, took a 1-0 lead on a goal by Baggio Husidic in the game's 14th minute, and opened the lead to 2-0 on a 74th-minute score by Marco Pappa.
Mwanga, who entered the game as a substitute at the start of the second half, scored his third goal of the season on a give-and-go with Sebastien Le Toux.
Nowak was not happy with Chicago's tactics.
"I think we were surprised from the minute the game started," he said. "The team that was supposed to play attractive soccer, attacking soccer at home, played with 10 guys in the back, behind the ball. I believe that in our league this stuff is not supposed to happen."
Still, the Fire outshot the Union, 6-2, with Chris Seitz making four saves in the Union net.
The Union (2-6-1) plays in Kansas City on Thursday night, before the league shuts down for a 2-week World Cup break. The team's first game after the break will be the first in its home PPL Park, against Seattle on June 27.