The three-time defending Liga champions are unbeaten on the domestic front so far this season, but have failed to carry that form into Europe and have won just once in Group G so far.
All is not lost as Porto sit just one point behind second-placed Zenit St Petersburg, with the top two progressing to the last 32, but Fonseca knows the importance of Tuesday's match.
"Porto go into every match wanting to win and it is imperative we do so," he told a press conference.
"At the moment things are quite clear; if we want to think about qualifying for the next round we have to win the game with Austria Vienna.
"We had trouble beating this same opponent in Austria. They are a team on the up, with three wins in their last three league games, and who defend well. But we have an obligation to win here and we are motivated."
Porto, who wrap up their group campaign with a trip to already-qualified Atletico Madrid next month, have doubts over Algeria striker Nabil Ghilas, who has been receiving treatment on a muscle injury.
Austria's chances of progressing are even slimmer as they sit three points adrift at the foot of the table.
However, three successive wins in the Bundesliga - including a 5-0 hammering of Wacker Innsbruck at the weekend - have done wonders for confidence, according to coach Nenad Bjelica.
"We have talked about our form a lot within the squad and a crisis always gives you the chance to improve things," he said.
"I think we have become a lot stronger."
Bjelica is also hoping striker Philipp Hosiner can continue his scoring form after netting a hat-trick in the weekend win.
He said: "We have seen in Innsbruck how important Philipp is for the team and how much we also depend on his goals."