CSO Association President Deborah Rutter said Friday that the 69-year-old conductor is undergoing tests to determine the underlying cause of his fall. She says Muti will have surgery Monday at Northwestern Memorial Hospital in Chicago.
Muti fainted and fell from his podium during a double rehearsal Thursday. Other conductors are filling in during Muti's absence.
In October, Muti withdrew from his fall residency in Chicago and returned home to Italy after suffering from "extreme gastric distress."
Orchestra officials said then that tests determined he was suffering from extreme exhaustion that resulted in abdominal pain and other symptoms. Doctors recommended a month of rest.