The Borussia Dortmund star limped off the field with tears in his eyes just before the break following a heavy challenge from Artur Yedigaryan at the Coface Arena in Mainz.
Reus was a certainty to start in his country's World Cup opener against Portugal next Saturday, but is now at real risk of missing Fifa's flagship event.
"He's in the hospital, undergoing a MRI scan," Low told ZDF. "We have to wait for the results. In such a situation you have many thoughts running through your head.
"During half-time we saw that his foot had swollen. I hope that he hasn't suffered a ligament injury and everything is okay.
"But it looked bad. We can call up another player until 24 hours before the first game."
Reus's teammate Mats Hummels is expecting the worst.
"Maybe you must expect the worst," the defender reasoned.
"Otherwise you could be surprised in a negative way."