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.
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.
"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."
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".
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.
