With only one club from each three-team group advancing and two tough road games ahead, the coach says getting maximum points at BMO Field is crucial.
"We've got to win the home games," Mariner said this week. "They're all difficult games away from home, but we have to take care of this one."
Toronto is languishing at the bottom of the standings in Major League Soccer with a 5-12-4 record, but has seen a resurgence of sorts under Mariner, who is 4-3-4 since taking over from Aron Winter.
The team has switched formations to a more direct 4-4-2 from Winter's Dutch-influenced attack and has seen better results.
"It's a question of us imposing our power and our system on the opposition," said Mariner, whose team lost 2-0 at home to Houston over the weekend. "We won't change our style (on Wednesday)."
Games against Central American opponents can sometimes degenerate into shenanigans away from the ball, but the Englishman adds that his players will be ready.
"You can't get involved in that sort of stuff. You've just got to hope that referee is strong and that the referee can see through some of the antics and not get wrapped up in it," Mariner said. "The players have been around the block. They know what to expect."
The MLS playoffs might be an unrealistic goal at this point for Toronto FC, but with another crucial league game on tap over the weekend, every match is being viewed as a must win.
"We've got to look at all our games as Cup finals," Mariner said. "Every single game is of vital importance to myself and the players — we just want to give a good account of ourselves."
Ironically, one of Winter's only successes this season was qualifying Toronto for the Champions League by way of winning Canada's domestic championship.
Toronto advanced to the semifinals of last year's CONCACAF competition, which covers North and Central America and the Caribbean, including a stunning defeat of MLS champion Los Angeles in March.
But the wheels fell off after Toronto lost in the semis over two legs to Mexico's Santos Laguna, which is also in TFC's group this time around, eventually forcing Winter to step down after an 1-9-0 start to the MLS regular season.
"When I took over we wanted to have steady improvements," Mariner said. "I think we've done that."
Aguila enters Wednesday's game in Toronto off a pair of league wins and sits fourth in the standings after three games.
Toronto hosts Santos, the Mexican champions, on Aug. 28, before playing at Aguila in San Miguel on Sept. 25. The Reds then visit Santos in Torreon, Mexico, on Oct. 24.
Note: Toronto FC announced the signing of goalkeeper Freddy Hall on Tuesday after a three-week trial with the club. "Freddy brings depth to our goalkeeping position and has many qualities that we like to see at this position," Mariner said in a release. "He performed well during his trial period with our club and played well against Liverpool."