The Germany attacker will miss two weeks of action with a muscle strain, while Sven Bender also picked up a knock in the 5-1 win over Werder Bremen
The duo started the 5-1 Bundesliga win over Werder Bremen on Saturday, but were both withdrawn after 65 minutes.
And the club have now confirmed that Reus had suffered a muscle tear, which will force him to miss the two upcoming games against Eintracht Frankfurt and the trip to Hamburg.
Bender, meanwhile suffered a strain in his right thigh which means he will be absent from the Pokal quarter-final at the Commerzbank Arena and next Saturday's meeting with Armin Veh's side.
Dortmund, who had been struggling for form in the Bundesliga, thrashed Bremen at the Weserstadion thanks to two goals apiece from Robert Lewandowski and Henrikh Mkhitaryan and a Manuel Friedrich strike.