Left winger Reus suffered a torn right thigh in Saturday's 5-1 Bundesliga hammering of Werder Bremen and will be out for a fortnight, while defensive midfielder Bender has a thigh strain and misses the next week.
But Dortmund coach Jurgen Klopp said the Bremen win meant Borussia headed to Frankfurt in buoyant mood after Poland striker Robert Lewandowski and attacking midfielder Henrikh Mkhitaryan both scored twice.
"From the first seconds, we saw what we want to see every week," said Klopp, with Dortmund third in the league and set to face Zenit St Petersburg in the last 16 of the UEFA Champions League. "That was a game where there was nothing to criticise."
Eintracht's Borussia double
In a double-header, Frankfurt host 2012 German Cup winners Dortmund in the cup on Tuesday, then travel to Borussia's Westfalenstadion on Saturday for a league match.
The double injury blow is more bad news for Klopp who lost Germany defender Mats Hummels last week after the centre-back injured his foot in a friendly and will miss the Frankfurt double-header.
Klopp is already without both centre-back Neven Subotic and winger Jakub Blaszczykowski for the rest of the season as the pair have torn knee ligament injuries.
Frankfurt also have a few injury concerns with captain Pirmin Schwegler carrying a rib injury and fellow midfielder Johannes Flum, who scored in Saturday's 3-0 win over bottom side Eintracht Braunschweig, has a thigh strain.
"We normally do well against Borussia at home, we lost 2-1 to them earlier this season and drew 3-3 last season, which was a real spectacle," said Eintracht midfielder Alexander Maier.
"If we put in our best performance, we make it tough for any team to beat us and we'll try everything to pull off a shock."
Bayern eye more silverware
European champions Bayern travel to Hamburg in Wednesday's ties while Bayer Leverkusen are home to second division Kaiserslautern, in the only non-Bundesliga clash, and Hoffenheim are home to VfL Wolfsburg.
Bayern, who are on a record 45 match unbeaten league run, are peerless in the Bundesliga and are on course to repeat last year's treble of European, league and cup titles.
Hamburg are in chaos having been booed by their own fans on Saturday after crashing to a sixth-straight league defeat, a club record, on Saturday as they were routed 3-0 at home by Hertha Berlin.
Coach Bert van Marwijk has been given a job guarantee by director of sport Oliver Kreuzer until at least Saturday's Bundesliga match at bottom side Eintracht Braunschweig, but first come Bayern in the cup.
Pep Guardiola's Bayern have already beaten Hamburg 3-1 in Munich last December while the European champions warmed up for their Hamburg Sojourn with a 2-0 win at Nuremberg on Saturday.