Xavi says Porto match represents 'turning point' for Barcelona
The Catalans missed the chance to qualify from Group H with two games to spare when they suffered a shock 1-0 loss to Ukraine's Shakhtar Donetsk in Hamburg at the start of November.
However, a victory over second-placed Porto at home will send Barcelona through to the knockout phase and lift some of the tension after a stuttering run of form over the past month.
"It's a very important match, it's vital. If we win we would be assured of finishing in the top two, after two years without being able to qualify for the last 16," Xavi said at Monday's pre-match news conference.
Barcelona won the last of their five European titles in 2015 in Xavi's final season as a player at Camp Nou.
They failed to make it past the group stage each of the last two seasons, dropping into the Europa League on both occasions, and there is renewed concern following the club's worrying run of underwhelming performances.
A 1-1 draw at Rayo Vallecano on Saturday left Barcelona four points adrift of La Liga leaders Real Madrid, while also trailing Girona and Atletico Madrid.
"Lately, things have not been going the way we want but we are on the right path. We believe in what we are doing," said Xavi, who has faced heavy criticism in Spain for the nature of his team's displays.
"We need to be very demanding of our ourselves and be self-critical, because we're no longer at the same level we were a month and a half or two months ago.
"Tomorrow is a great opportunity. We can confirm our place in the round of 16 after five matches. It could be a turning point."
Barcelona are without teenage midfielder Gavi for the rest of the season after he sustained a devastating cruciate ligament injury while on Spain duty.
Goalkeeper Marc-Andre ter Stegen is also a doubt for the Porto game after he hurt his back while away with Germany.
"Ter Stegen will be tested tomorrow to see how much he's suffering and if he's ready for the match. He's virtually not trained with the group but he's always there to bring something extra to the team," said Xavi.
