Roberto Di Matteo's side were thrashed 5-0 in front of their home fans by Chelsea on Tuesday - a stark reminder of just how much work the Italian has to do at the Veltins Arena.
Schalke dropped to third in Group G as a result and now need to win in Maribor - and hope Sporting Lisbon lose at Chelsea - on matchday six to progress.
The big home defeat went against the trend of Di Matteo's short time in charge of Schalke - that being wins at home and losses on the road.
Di Matteo had led Schalke to three straight home wins before the Chelsea loss, but is yet to pick up a point on his travels.
So the coach - who led Chelsea to Champions League glory in 2012 - will surely welcome another home fixture, with this one coming against the ninth-placed Mainz, who have not won in four.
Schalke sit seventh themselves and are only three points adrift of third.
Leaders Bayern Munich were also beaten in Europe's premier club competition on Tuesday, conceding twice in the last five minutes to lose 3-2 at Manchester City.
The rare defeat is sure to leave Pep Guardiola's men angry ahead of their visit to the German capital to face Hertha Berlin.
Bayern are the only team yet to lose in Bundesliga this season and have scored 31 goals in their 12 matches.
Guardiola's side lead the league by seven points and will hope attacker Mario Gotze - the league's joint top-scorer with seven goals - can continue his excellent start to the campaign.
Frankfurt's Alexander Meier is the other player in Bundesliga with seven goals this term.
Meier is in action against Borussia Dortmund on Sunday, as Jurgen Klopp's men look to continue their rise up the table.
Dortmund still sit 16th - four places blow Frankfurt - but could climb to 13th with victory.
Also on Sunday, Bayern's nearest rivals, Wolfsburg, will look to respond to a defeat at Schalke when they host the third-placed Borussia Monchengladbach.
The two sides are separated by just three points.
Bayer Leverkusen, who sit fourth, host Cologne in a local derby on Saturday, while Werder Bremen face Paderborn and Hoffenheim take on Hannover.
Augsburg and Hamburg also meet on Saturday, while Friday's fixture sees rock-bottom Stuttgart go to Freiburg.
