Keller's side have made a poor start to the Bundesliga, failing to win any of their first three matches.
Patchy league form was compounded by a 1-1 home draw with PAOK in the first leg last week, giving Schalke a tough task in Northern Greece to make the group stages.
The under-fire boss admitted that his side are under huge pressure to turn the tie round, but can draw on the experience of reaching the knockout stages last season.
He told reporters: "This is a very important match for the club, for everybody at Schalke. The pressure would be huge in any case, regardless of our run in the league.
"The 1-1 first-leg result is not ideal, but it is not fatal. We experienced the inverse scenario last season when we drew 1-1 with Galatasaray in Istanbul and then got eliminated at home.
"We have worked hard to reach this point. If we make it to the group stage, the team stand to benefit a lot from their participation in the Champions League."