In response to repeated abusive chants from a section of the crowd during a friendly game against the fourth-tier club on January 3, the former Tottenham and Portsmouth player kicked the ball into the stands before leading his team-mates off the pitch.
Boateng's actions resulted in the abandonment of the match and sparked fresh debate on the problem of racism in football, with the 26-year-old set to meet FIFA president Sepp Blatter in Zurich on Friday to discuss the issue further, having criticised the governing body for not doing more to tackle it.
The Gazzetta dello Sport on Wednesday reported how Boateng told judge Toni Adet Novik at a court in Busto Arsizio he had suffered similar abuse during his time in Germany, where he represented both Hertha Berlin and Borussia Dortmund.
He said: "Every time I touched the ball I could hear chants being directed straight at me - a kind of 'buh buh' noise that you'd make at an animal.
"I think they were insulting me because my skin isn't white. It happened in Germany too and for me this is clearly racism."
Milan head coach Massimiliano Allegri and full-back Daniele Bonera, who captained the Rossoneri during the ill-fated match, also testified against the accused Pro Patria fans.
"He was very shocked and saddened," Allegri said. "He was not happy over the following days.
"These are serious incidents which must not happen and the decision to interrupt the match was the right one."
Milan players Sulley Muntari, M'Baye Niang and Urby Emanuelson - who is currently on loan at Fulham - were also targeted, according to Bonera.
"He was being insulted every time he touched the ball and the (black) players were very upset," he said.
"The chants reminded me of monkey noises and I think he's intelligent enough to know the difference between a racist chant and normal teasing."