Down a man for the entire second half, the Impact nearly held on for a draw in Guatemala, but Heredia snatched a goal in the dying minutes of the game.
Following several chances for the home side to pull in front, including a penalty which was smashed off the crossbar in the 17th minute by Cordoba, the Impact escaped the first half unscatched. But Montreal was, however, forced to go down a man just before halftime, as Adrian Lopez was shown a red card for a late challenge.
Despite playing a man down for the entire second half, the Impact had a great chance to find the breakthrough in the 73rd minute through midfielder Blake Smith. After flicking a Daniele Paponi pass over Heredia defender Robert Arias, the 22-year-old sent a blistering volley towards goal but only to see it rattle off the crossbar.
With a few minutes left before full time, it seemed like the Impact would manage to hold on for the draw, but the home side found a winner. David Espinoza sent in a low cross into the box that Montreal goalkeeper Evan Bush bobbled in front of him and Cordoba easily finished from close range.
The Impact will play the San Jose Earthquakes in California on Sept. 17 before locking horns with Heredia a week later in the final game of group stage. Heredia are now tied with the Impact in first place on three points.