Spain international Martinez was taken off on a stretcher with his knee heavily bandaged after an attempted bicycle kick with Dortmund defender Marcel Schmelzer marking him in the 29th minute.
Sammer told reporters there were fears it could be a cruciate ligament injury that could keep the player out for some time but the club has not yet announced the full extent of his injury.
The Bundesliga season starts next week.
- Sports & Recreation
- Marcel Schmelzer
- cruciate ligament injury