Andrea Conti is scheduled for a second operation after spraining the same left knee that required surgery in September.
The 24-year-old has spent the majority of his first season at AC Milan on the sidelines because of an ACL tear. He then suffered a setback in training last week as he entered the final stage of his recovery.
Milan announced Wednesday that further tests revealed "instability" in his knee, with Conti flying out to the United States for consultation.
He's now expected to return in time for the beginning of the 2018-19 campaign.
"We're all sad about it," AC Milan sporting director Massimiliano Mirabelli, who made Conti one of his first orders of business last summer, told Milannews.it. "He's an important player for us and a great lad. I'm sorry for his morale levels, but it's part of the game. He'll be our first signing for the next season."
Conti, who's made just five appearances for the Rossoneri in all competitions, joined Milan by signing a reported €23-million deal last July after impressing for Atalanta as a right wing-back.