The Greek side have a chance of reaching the group stage of the competition for the first time after they drew the first leg 1-1 in Germany last week.
The return, however, will be played behind closed doors at their Toumba stadium as PAOK are one match into a three-game supporter ban handed to them by governing body UEFA for crowd violence during a Europa League tie with Rapid Vienna last year.
"It will be a very difficult game, especially without our fans it will seem like a friendly and we saw how tough it was in our previous match against Metalist in this situation," Katsouranis told Greece's Novasport channel.
"We have to be 100 percent focused and disciplined in order to overcome the fact that we won't have our fans with us."
PAOK were beaten by Metalist Kharkiv 3-1 on aggregate in the third qualifying round but were reinstated in the competition after the Ukrainian club were disqualified over their involvement in a domestic match-fixing case in 2008.
There will be an extra incentive for PAOK manager Huub Stevens to finish the job and eliminate their German opponents after he was sacked by Schalke last season, despite leading them to the knockout stages of the Champions League.
Katsouranis, who has more than 100 caps for Greece, is confident PAOK can make it through if they repeat their first-leg performance when Miroslav Stoch cancelled out Jefferson Farfan's opener for Schalke.
"It's a historic match for us as it gives the club the opportunity to play in the group stages of the Champions League for the first time," Katsouranis added.
"Schalke are a very good team but after the result of the first match I think that if we can repeat that performance then we should be able to qualify."