The 21-year-old Germany international had to be carried off on a stretcher after 20 minutes following his horrific injury when his foot got caught in the grass and snapped back.
"He told me he felt it immediately," said Dortmund coach Thomas Tuchel. "It cracked in there. The first diagnosis is that it is a broken ankle."
"This is a bitter loss for us which we cannot compensate one-to-one currently. We will try to find a solution for the last two matches of the season which will be very difficult.
Dortmund are third in the Bundesliga on goal difference with a game left to play as they battle to hold on the their spot that leads to the Champions League group stage next season.
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The Ruhr valley club host Werder Bremen in the season finale next week and then take on Eintracht Frankfurt in the German Cup final on May 27.
"Now the focus is on the two finals we have ahead of us. Everything else needs and will be blocked out," Tuchel said.
(Reporting by Karolos Grohmann, editing by Pritha Sarkar)
