The Germany international - plagued by ankle problems in 2014 - was forced off midway through the first half of Tuesday's DFB-Pokal victory at Dynamo Dresden after colliding with Dennis Erdmann.
While it sparked fears over Reus potentially facing another lengthy layoff, coach Jurgen Klopp initially explained the injury was "only a dead leg".
A subsequent statement confirmed Reus had only sustained bruising "above the knee" but, with the problem now affecting a wider area, his involvement in this weekend's Bundesliga fixture is uncertain.
"Reus' bruise is currently spread over the entire thigh," Klopp told a news conference on Thursday.
The Dortmund coach also offered an update on Pierre-Emerick Aubameyang and Nuri Sahin, who both sat out the cup clash due to minor injuries.
"Aubameyang will train tomorrow," he added. "With Sahin it seems unlikely, but you never know."
