The Borussia Dortmund player went down in the 43rd minute as he chased a ball and then covered his face with his hands as he was helped off the pitch.
"I do not know how bad it is," coach Joachim Loew told ZDF television. "He is being examined as we speak in hospital and we will wait and see what comes out of it.
"A lot goes through your head at that moment and you just hope it is not too bad but then you see it swell up a bit. I hope the ligaments hold up.
"Until 24 hours before the first World Cup game, according to the rules, we can nominate someone else."
Germany, who take on Ghana, Portugal and United States in Group G, leave for Brazil on Saturday.
The lightning-quick Reus, who enjoyed a superb season at Dortmund, is a key player in Germany's quest to claim their first international title in 18 years.
- Sports & Recreation
- Borussia Dortmund
- Joachim Loew