The attacker says injuries are to blame for his side's struggles before the winter break but is optimistic they will improve
Dortmund have dropped to fourth in the league after just one win in their last six Bundesliga encounters and are 12 points behind leaders Bayern Munich in the title race.
Injuries have hampered Dortmund's progress this season, with the likes of Neven Subotic, Mats Hummels and Ilkay Gundogan all missing much of the season.
Still, with Hummels and Gundogan poised to return, Reus is hopeful that Dortmund can recover the form that saw them reach the Champions League final last season when some of their key players regain fitness.
"We have shown strong performances in the Champions League and also in the DFB-Pokal," Reus said.
"And, despite an unprecedented series of injuries we remain within striking distance in the Bundesliga.
"If our injured return and we show our quality, we are second."
Dortmund resume their Bundesliga on January 25 when they host Augsburg at Signal Iduna Park.