The 25-year-old Borussia Dortmund attacking midfielder had only recently come back following his injury in Germany's final World Cup warm-up game against Armenia in June.
"Reus turned his ankle again and it is the same one as the one against Armenia," Loew told reporters.
"Initial diagnosis suggests it may not be as bad as before the World Cup. But we will have to wait for more checks."
Reus, a transfer target for almost every top European club, went down in stoppage time of Germany's first competitive game since their World Cup victory, grimacing in pain, before being helped off the pitch.
Last season's runners-up Borussia Dortmund kick off their Champions League group matches against Arsenal on Sept. 16.
(Reporting by Karolos Grohmann; editing by Martyn Herman)
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