The Impact will attempt to become the first Canadian team to reach the CONCACAF Champions League final when they face Alajuelense on Tuesday night in Costa Rica.
Montreal holds a 2-0 lead in their two game, total goals semifinal thanks to a victory in the opening leg March 18 at Olympic Stadium on goals from Ignacio Piatti and Victor Cabrera.
But they are aware of how quickly the lead can disappear without solid defensive play against a talented attacking team. In Major League Soccer last week, they led Orlando 2-0 after 14 minutes only to give up two goals in a two-minute span.
And Alajuelense is expected to go on all-out attack from the outset.
"They've got to push the game, try to get a goal and get momentum on their side with their fans behind them," assistant coach Mauro Biello said Monday. "We have to manage the storm early on and then try to take our chances.
"If they push up in numbers it'll leave space for us and we have to take advantage of that."
The Impact use the Orlando match as a lesson in staying vigilant on defence because it is recent. Biello didn't want to think about the 2009 CONCACAF quarter-finals, when they won the first leg 2-0 and then went to pieces late in the return leg at Santos Laguna in Mexico, losing 5-4 on aggregate.
Alajuelense, led by former Vancouver Whitecaps striker Jonathan McDonald, is taking it seriously. They used only two of their regular starters in a 3-1 loss to Cartagines in league play on Saturday, resting most of their top players for the semifinal. A large crowd is expected at Estadio Alejandro Morera Soto in Alajuela, Costa Rica.
The Impact got a break when MLS moved a match scheduled April 4 against the New York Red Bulls to October to leave them fresh for the semifinal.
They hope to be only the second MLS team, after Salt Lake in 2011, to reach the CONCACAF final since the Champions League started in 2008-09.
Montreal should have a full lineup, with Belgian defender Laurent Ciman and midfielder Maxim Tissot of Gatineau, Que., back from national teams duties with their respective countries. Midfielder Dilly Duka, injured early in the first leg, has been judged fit to return.
Duka scored twice against Pachuca in Mexico in the quarter-finals.
Out with long-term injuries are midfielder Justin Mapp (fractured elbow) and forward Cameron Porter (torn ACL).
While the Impact have a game Saturday in Houston, they will use their best available 11 in the Champions League semifinal, which at this stage dwarfs league play in their estimation.
"It's the biggest game in our history," said Biello. "That's our mindset: this is a moment that may not come back and we have to seize it. That's without putting too much pressure on. It's the reality."
It remains to be seen if they use Jack McInerney at forward or move the speedy Dominic Oduro up front so that both Duka and Andres Romero can play as midfielders, along with Piatti.
Evan Bush is to start in goal. The Impact picked up John Smits, the starter for FC Edmonton of the NASL, on loan to be back-up for the match after returning Kristian Nicht to the Indy Eleven.
Regular back-up Eric Kronberg is ineligible because he played in the current Champions League campaign for Kansas City, while third stringer Maxime Crepeau has a shoulder injury.