Hector Villalba and Jeff Larentowicz each scored, Brad Guzan had his second straight shutout, and Atlanta beat the Montreal Impact 2-0 on Sunday to extend United's home unbeaten streak to 12 games.
Atlanta (14-8-7) moved into third in the Eastern Conference, three points back of playoff-clinching New York City FC. Montreal (11-13-6) trails the New York Red Bulls by three points for the final playoff spot.
"They put the pressure on at the start of the game and they were able to score in the first half," said Impact head coach Mauro Biello. "We created some chances to equalize, but we did not take those opportunities to get the result we wanted on the road. In the end, details like this make the difference."
Villalba opened the scoring in the 28th minute with a long-distance shot for his 12th goal of the season. Larentowicz scored his first goal for Atlanta in the 73rd by knocking in Julian Gressel's cross.
Gressel's eighth assist of the season is the second most for a rookie in MLS history.
Montreal goalkeeper Evan Bush faced intense pressure for much of the match. On the offensive side, the Impact's best scoring chance came in the 72nd minute on Michael Salazar's close-range shot on a breakaway but Guzan stayed low to defend the near post.
"We did pretty well during some stretches. We had our chances to level the game and we did not take advantage of those," said Bush, who ended the game with five saves. "A team like that will make you pay. They are a very good team, probably in the top two in the league. We have to rebound and get ready for Wednesday."