The Borussia Dortmund ace was forced off just before the break in the game at the Coface Arena in Mainz, after collapsing to the ground in agony clutching his ankle following a heavy challenge from Artur Yedigaryan.
Given Reus was a certainty to start for Germany in their World Cup opener against Portugal, Low was understandably left deeply concerned by the attacker's injury-enforced withdrawal.
"He's in the hospital, undergoing a MRI scan," the coach 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."
Mats Hummels, meanwhile, believes it would be best for Germany to consider Reus' World Cup over in the hope that they are pleasantly surprised.
"Maybe you must expect the worst," the defender reasoned. "Otherwise you could be surprised in a negative way."
Germany, who also have USA and Ghana for company in Group G, face Portugal in Salvador on June 16.