Nmecha got hurt during a challenge and had to be substituted just 12 minutes into Saturday's 2-2 draw against Werder Bremen.
The 24-year-old, who came up through Manchester City's youth system, has been one of the few players at Dortmund who hasn't faced much criticism during the team's recent struggles.
Dortmund is set to face Shakhtar Donetsk in their last Champions League group stage match on Wednesday.
Despite winning only one of their last 10 matches across all competitions, they still have a shot at qualifying for the top eight if they win and other results favor them.
“Clearly it’s not a simple situation but in every club there’s difficult phases and the job is to overcome these challenges,” goalkeeper Gregor Kobel told reporters on Tuesday.
“That’s what makes a great team… We just need to chalk up some wins and to experience a bit of success again.”