If they win their meeting on Tuesday night, the Impact will be in the driver's seat when they close out group stage play Sept. 24 at home against CD Heredia of Guatemala.
If they lose, or even draw, their future in the tournament becomes dicey.
"It's going to be tough," said midfielder Davy Arnaud. "It's a long trip, a long flight and they're a good team.
"But we're ready. We know how big Tuesday's game is."
CD Heredia leads the group with six points, posting 1-0 victories at home against both Montreal and San Jose. The Impact is second with three points from a 1-0 home win over the Earthquakes. San Jose has two losses, but has yet to play at home.
The first-place team advances to the quarter-finals in March.
It will be a test on many levels for Montreal, which will play only three days after blowing a lead in a 2-1 defeat at home to Columbus in Major League Soccer play.
There is the six-hour flight to the west coast to contend with, and jet lag may be a factor as the game starts at 10 p.m. ET.
One question is how many veterans coach Marco Schallibaum will use after fielding all of his first 11 against Columbus, and how much 30-somethings like defenders Matteo Ferrari and Alessandro Nesta, midfielder Patrice Bernier and striker Marco Di Vaio will be able to play.
When naming his lineup, Schallibaum must also consider that they have a league game to play Saturday at home against the Vancouver Whitecaps.
Impact management wants to do well in the Champions League. They recall the international recognition the team got when it reached the quarter-finals in 2009 and drew 55,571 fans to a match against Mexico's Santos Laguna at Olympic Stadium.
"The club definitely puts a lot of emphasis on winning it," said midfielder Collen Warner. "It does great things for the club _ a lot of good press, a lot of exposure.
"So they definitely want to do well in it."
It is for games like this that the loss through injuries last week to mid-season signings Hernan Bernardelli and Adrian Lopez hurts most.
The Spaniard Lopez should have been ready by now to step into the central defence to replace Nesta or Ferrari, while Bernardelli could have kept another midfielder fresh. That central defender Nelson Rivas has not played yet this season due to a knee injury leaves even less depth at that key position.
Players mainly used as substitutes, like midfielders Andrea Pisanu, Andres Romero and Blake Smith, and forwards Daniele Paponi, Sanna Nyassi and Andrew Wenger could see action. Backup Evan Bush normally plays in goal instead of Troy Perkins in non-MLS matches.
The team's workhorses, fullbacks Hassoun Camara and Jeb Brovsky, will no doubt play as they always do.
A bigger question may be what lineup San Jose puts on the field. The Earthquakes are second-last in the Western Conference at 10-11-8, and may see advancing in the Champions League as a way to make up for a weak season in league play.
That was Bernier's concern.
"We know their roster a little more and we've seen them play in the league a lot more (than CD Heredia), but we don't now what their approach will be," Bernier told the Impact's website. "They are in a different position than us in the standings, so they might come out full force, putting the focus on the Champions League."
The Impact (13-8-6) is second in the MLS East and has a good shot at making the playoffs in only its second year.
The club reached the group stage by beating Toronto FC and Vancouver to win the Amway Canadian Championship.
"It's a different style of game," Warner said of the non-league matches. "And with it being a mid-week, night game, it takes on a different play to it.
"We have a few days off to recover. I think it will be nice to get a switch out of the playoff chase into CONCACAF."