The midfielder notably walked off the pitch in a friendly against Pro Patria in June, while former teammate Kevin Constant was forced to do the same against Sassuolo a month later as a result of a torrent of racially aggravated chants from the stands.
The Ghana international joined Schalke this summer for 10 million euros, and Peters has revealed the move came as a result of a previously agreed arrangement with Milan over his future.
"Because of the racist incidents, Boateng wanted to leave Italy," Peters told SportBildPlus. "He had an arrangement with president Silvio Berlusconi that he could leave if AC Milan qualified for the Champions League group stage."
Boateng signed a four-year deal at Schalke the day before making his debut against Bayer Leverkusen.
Milan reached the Champions League group stage following a 4-1 aggregate playoff win over PSV, while Schalke triumphed over PAOK to reach the tournament proper as well.