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 German 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, who played in the Premier League for Tottenham Hotspur and Portsmouth, joined Schalke from AC Milan in August and has since made 17 appearances in all competitions, scoring six goals.