German newspaper Bild claims the former Tottenham midfielder sustained injuries to his ribs, back and neck after he was attacked in a street in Kaarst, near Dusseldorf.
A police statement 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 also previously played for AC Milan, is understood to have headed home after the assault before admitting himself to hospital.
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