Ahead of Saturday's fixture at the RheinEnergieStadion, both Reus and Gundogan have been in full training with Klopp confirming the pair could return to full competitive action this weekend.
Gundogan has not featured since August last year due to ongoing back problems while Reus suffered a second ligament injury in four months while on international duty with Germany in September.
Both took part in an Under-23 match on Friday and are ready to boost a Dortmund side who have taken just one point from their last possible 12 in the top flight.
Henrikh Mkhitaryan is also available after a foot injury while Sven Bender and Sebastian Kehl are fit again after thigh and groin problems respectively.
Shinji Kagawa and Ciro Immobile are likely to shake off knocks from the international break ahead of Saturday's trip to Cologne but Klopp warned the return of some star names does not mean Dortmund can take anything for granted.
"Marco, Kehl, Mkhitaryan and Ilkay are all training regularly and will be available," he said.
"[Reus and Gundogan] are able to play the football Dortmund demand [but] we must not believe that just because other names appear on the team sheet, we automatically win games.
"There is nothing to worry about Kagawa's concussion. He trained yesterday. Erik Durm has a light cold from his trip with the national team, everything's okay with Mats [Hummels].
"Immobile got a knock on his calf, but that also shouldn't be a problem."
