The Italian coach believes his side still have every chance of progressing to the next round, despite their unimpressive start to the competition
The Russian giants have collected just one point from their opening two games, but Spalletti is confident they can still make it to the next round.
"The players believe they can qualify and I know they have the ability to get a decent result," Spalletti said at a press conference.
"Porto are in very good shape despite their defeat by Atlético. We haven't started well but we have the quality to accumulate points.
"It may not be a critical game, but when you look at the table it's clear that Porto are a threat to second place. We have a fairly new squad though everyone is in high spirits and believes they can go far."
Tuesday's encounter will be a special occasion for Zenit striker Hulk, who will take on the club where he rose to prominence, and Spalletti is hopeful the Brazilian can make the difference.
"Hulk is a great player, very skilful, but can still improve. I know how much he loves Porto after the time he spent here. Tomorrow he just has to do what he always does: play at a very high level."