Dortmund slumped to a 2-1 defeat at Cologne on Saturday - their fifth loss in eight Bundesliga matches.
Coach Jurgen Klopp slammed his team for being "so naive" as they fell to 14th in the league table, already 13 points adrift of leaders Bayern Munich.
The UEFA Champions League has been a welcome distraction for Dortmund this season, though, and they top Group D with two wins from as many matches.
Klopp's side started their continental campaign with a 2-0 home success against Arsenal in September before they cruised past Anderlecht 3-0 last month.
Forwards Ciro Immobile and Adrian Ramos have netted twice each in the competition and Dortmund are five points ahead of the third-placed Galatasaray, heading into the Istanbul contest.
And Reus has called on Dortmund to keep working hard ahead of their trip to Turkey.
"We need to fight our way from the bottom as a team," Reus told Dortmund's official website.
"I have been through it before enough times, and it will not be easy.
"We need to keep working hard, and with that work I am sure that we shall soon be on the road to success again."
Reus returned from an ankle injury against Cologne, while Ilkay Gundogan made his first competitive start for over a year after recovering from a back injury.
Goalkeeper Roman Weidenfeller is pleased the games are coming thick and fast in the club's poor run and said after the Cologne loss: "It's good that we play again on Wednesday so that we can put things right."
Dortmund's Bundesliga plight was perhaps summed up when sporting director Michael Zorc said they are enduring their "most difficult time in years".
Things are going far smoother for Galatasaray on the domestic front, with three successive wins lifting them to second in the table.
And Cesare Prandelli's side will feel confident after a 2-1 win over Fenerbahce on Saturday, in which Wesley Sneijder scored two superb goals in the last five minutes.
Their Champions League form has been poor, though, a 1-1 draw at Anderlecht followed by a 4-1 loss at Arsenal.
But backed by a fervent home crowd, there is perhaps no better time to play Dortmund - something Galatasaray will be looking to take advantage of.
