Reus suffered the injury playing in Germany's FIFA World Cup warm-up match against Armenia last week, and was subsequently ruled out of the tournament in Brazil.
Initially, it was thought the forward would be out for a matter of weeks rather than months, but Dortmund have now announced that he is not expected to return before September - meaning he will miss the start of the 2014-15 Bundesliga campaign.
Further examinations by Dortmund's medical staff revealed Reus had suffered "a partial tear of the anterior ligament above the left ankle".
The 25-year-old will now begin rehabilitation on the injury, with Dortmund doctor Markus Braun expecting him to be unavailable for "a total of about three months".
Reus scored 23 goals in all competitions for Dortmund last season, and his absence represents a huge blow to Germany's chances at the World Cup.
