The Spanish side looked to be easing towards a last 16 place after winning their opening three games, but defeat to Shakhtar Donetsk in their previous match leaves progress still in doubt, having failed to emerge from the group stage in the last two seasons.
"It's a very important, vital game for us. It's up to us to be first," Xavi told reporters.
"After two years, it's a great opportunity. We're playing at home. We need the fans. Hopefully it's going to be a magical night.
"It's essential that the fans are there. The last few years, we have suffered. It's important that they are with the team."
Barca also failed to win their most recent LaLiga game, drawing 1-1 at Rayo Vallecano on Saturday, which saw them slip to fourth in the table, four points behind leaders Real Madrid.
"Lately, things aren't going our way, but the Vallecano game was there to be won," the manager said.
"We created play and in the second half we were more aggressive. We lack continuity in the game, but people are comfortable with the plan. The dressing room is united."
Xavi is also confident that he has the backing of the club president Joan Laporta and sporting director Deco, despite the recent poor results.
"I feel supported by the club in every way. I talk to the president and Deco every day," Xavi said.
"We have friendships. They believe in the project, they know how we work. I feel total confidence."
Barcelona are already without midfielder Gavi (19) after he tore his anterior cruciate ligament on international duty with Spain, and keeper Marc-Andre ter Stegen (31) missed Saturday's game with a back injury, but could return for the Porto tie.
"Tomorrow Marc will try, let's see how he is," Xavi said.
"He hasn't trained. Tomorrow we'll try to see if he's 100%."
Barca came away from Porto with a 1-0 win in October, and now host the Portuguese side on Tuesday and Xavi knows his team are in for another tough game.
"They're a team that works hard tactically, they press high and are aggressive.
"In Porto we suffered, they are a very strong opponent."
Barcelona lead Group H, level on nine points with Porto, while Shakhtar Donetsk are three points behind, and they face bottom club Antwerp on Tuesday.
