Christian Pulisic could be set for a period on the sidelines with “torn calf fibers."
The 20-year-old had played in four straight games for Dortmund, after missing two matches earlier in the season with muscle problems.
Pulisic is now in doubt for the USA's two upcoming friendlies against Colombia and Peru, set for October 11 and 16.
The American has played just one game for his country in 2018, after making nine appearances in 2017 as the U.S. shockingly failed to qualify for the World Cup.
Dortmund defeated Monaco 3-0 at Signal Iduna Park, making it two wins from two Champions League matches thus far.