Ryan Babel could be punished for publishing a derogatory tweet aimed at referee Thorsten Kinhofer after Hoffenheim's defeat to Hertha Berlin on Saturday.
The Dutch forward was dismissed after receiving two yellow cards in the space of two minutes on the stroke of half time. His team subsequently lost the match 3-1.
Babel, clearly disgruntled by the decision, announced on Twitter: "I don't know, I think the ref was on drugs."
The German Football Federation (DFB) is now investigating the 25-year-old, and its disciplinary committee has demanded a statement from the player.
While playing for Liverpool in January 2011, Babel received and £10,000 fine for posting a mocked-up image of referee Howard Webb in a Manchester United shirt on the social networking site.