Reus was hurt during Germany's 6-1 victory over Armenia on Friday evening and he was carried from the field of play just before half-time.
The Borussia Dortmund forward was caught in a challenge on his left ankle, which he seemed to get caught in the ground, and he was in tears as he left the pitch.
He was taken to hospital for scans on the injury, but there are fears that Reus will be forced to miss the World Cup in Brazil.
"I do not know how bad it is," coach Low 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 leave for Brazil on Saturday and face Portugal in their opening game on June 16 before also meeting Ghana and the United States in Group G.
The injury to Reus overshadowed an impressive second-half performance by Germany which also saw Miroslav Klose become the national team's all-time leading scorer.
Klose scored his 69th goal for Germany to surpass Gerd Muller's record and the Lazio striker was pleased with the landmark.
He said: "Of course, that means a lot to me, but more important was to be in form."