The alleged victim, identified as Miad J. by WXYZ TV, said the incident occurred at Cheetah's strip club Wednesday night after he attempted to speak with the musician, Allhiphop.com reported.
Miad told reporters that the rapper, whose real name is Marshall Bruce Mathers III, punched him repeatedly without saying a word to him.
"Eminem got done and boom," he said, "he started swinging. I wasn't even expecting it. I was just minding my own business, taking a leak."
Detroit investigators have yet to speak to the 33-year-old hip-hop star and, until the prosecutor's office receives all pertinent information, no charges will be filed.
Representatives from Cheetah's and from the rapper declined comment, the Web site said.