The Fire also happen to be the Union's next opponent after the Montreal game, Aug. 12 at PPL Park.
The Union insist they won't be looking ahead, because if they don't take care of business against the Impact, then the hole they have dug will get deeper.
Montreal (8-13-3, 27 points) is ahead of the Union, although the Impact have played five more games. The Union beat them at PPL Park on July 14, one of the Union's current four-game MLS winning streak at home, but it wasn't easy. The Union needed a goal by Carlos Valdes in the 91st minute to earn the 2-1 win.
"We know they will have a sour taste from that game," Union interim team manager John Hackworth said.
Hackworth, whose team is coming off Sunday's 2-1 win over the visiting New England Revolution, also realizes that the difference between the top and the bottom isn't always great.
Take last week, for instance, when Montreal, sitting in seventh place in the Eastern Conference, beat the visiting New York Red Bulls, 3-1. That was a Red Bulls team that had beaten the Union, 2-0, on July 21 at Red Bull Arena and entered the weekend in first place in the Eastern Conference. It's a team that has aspirations of winning the MLS Cup, and yet Montreal came out with the win.
"Things are very close in this league," Hackworth said.
Montreal is 7-3-2 at home, more evidence that this should be a difficult encounter.
The Union must take advantage of a Montreal defense that has been extremely generous. The Impact have allowed 43 goals, more than any other MLS team. True, Montreal has played more games than any MLS team, but that is still nearly double the total that the Union have surrendered (22).
Which brings us to one of the weaknesses of the Union - failing to cash in on scoring chances.
Just about any MLS team could claim to have that problem, but the Union frequently take one too many dribbles in the scoring area instead of unleashing shots.
Still, the Union are putting a lot of pressure on opposing defenses, and now that they are in the playoff picture, they are doing the same to teams ahead of them in the standings.
The recent close wins over Montreal and New England have provided more confidence, but not overconfidence.
"We are pretty confident that we can go there and get a result," Hackworth said.
Hackworth understands what his team is up against. The Union don't view Montreal as a typical expansion team.
Conversely, the Union have played much better than their record indicates, and opponents aren't viewing them as a losing unit.
This has the making of a game equal to the one last month at PPL Park. Regardless of the outcome, one can rest assured that the Chicago Fire won't enter the mind of either team until well after this one is in the books.
Contact Marc Narducci at 856-779-3225 or firstname.lastname@example.org, or follow on Twitter @sjnard.