Both players were sent to hospital during Bayer Leverkusen's 1-0 win at Signal-Iduna Park on Saturday, which was decided by Son Heung-Min's first-half goal.
The game was a feisty affair, with Leverkusen's Emir Spahic and Dortmund's Sokratis Papastathopoulos sent off, and both Bender and Sahin were forced off the field after tackles from their opponents.
Klopp's side have had a particularly tough time with injuries recently, with centre-backs Neven Subotic (knee) and Mats Hummels (ankle) both long-term absentees.
Defenders Lukasz Piszczek and Marcel Schmelzer have also been absent, although the former did feature off the bench against Leverkusen.
And Klopp revealed that both Sahin and Bender were in extreme pain and were highly unlikely to feature against Marseille in the UEFA Champions League on Wednesday.
"Today is Saturday and while miracles do happen, they just haven't happened for us recently," Klopp said.
"This is not an easy phase for us. Given that two of our players are injured, possibly gravely so, it will make our next few games tricky."
Dortmund's fourth Bundesliga defeat of the season leaves them in third, only goal difference behind Borussia Monchengladbach.
They are already six points behind the second-placed Bayer Leverkusen, and 10 adrift of runway leaders Bayern Munich.
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