The Portuguese champions sit third in Group G, one point behind second-placed Zenit St Petersburg with two matches left to play.
And, with Zenit hosting group winners Atletico Madrid on Tuesday, Fonseca believes his side need a victory over their Austrian opponents ahead of visiting the Spanish capital on the final matchday.
"At the moment, things are quite clear," he said. "If we want to think about qualifying for the next stage, or think about going to Madrid to compete for a place in the next phase, we have to win the game with Austria Vienna.
"We had trouble winning in Austria (1-0 on matchday one). This is a team on the rise, with three wins in three games (in the Bundesliga), which defends well."
But Fonseca's focus will be solely on his own team's performance, with little of his attention on the group's other tie.
"We need to focus on winning this game and then see what happened to our opponents," he continued. "Even with all absences, I believe Atletico will go to Russia with the clear intention of winning.
"We could be better than we are, but we remain in a strong position in all competitions in which we compete."
Porto currently sit top of the Primeira Liga and are unbeaten in the league this season.
