The Ghana international, 26, suffered injuries to his ribs, back and neck following the incident in Kaarst - a town near the city of Dusseldorf.
A statement released by police read: "On Wednesday, December 25, 2013, at around 12.45, an incident of physical assault took place in Kaarst.
"A 26-year-old man, a resident of Meerbusch, was physically attacked on the street by a currently unidentified man. There are indications that the motive for the crime is connected to the personal affairs of the 26-year-old.
"The man was mildly injured. Criminal investigators have begun their work."
Boateng joined Schalke from AC Milan in a 12 million euro deal in August and has since made 17 appearances in all competitions, scoring six goals.