McMillon, a fifth-year senior from Memphis, will be sidelined for an "indefinite period," Saban said. Al.com previously reported that McMillon might miss the entire 2019 season.
"We have a great medical staff here and he will have the full support of everyone in the organization as he goes through the rehab process," Saban said in a statement. "We are confident Josh will make a full recovery."
McMillon played sparingly on defense in his first three seasons, finishing with 14 tackles and one tackle for loss in 10 games in 2018. But he was projected to start next to returning inside linebacker Dylan Moses, a 2018 Butkus Award finalist.
Sophomores Markail Benton and Ale Kaho will probably be next in line to try to win the job. The Tide bring back outside linebackers Anfernee Jennings and Terrell Lewis, who missed the entire 2018 season with a knee injury.
The Crimson Tide, who open the season against Duke in the Chick-fil-A Kickoff Game in Atlanta on Aug. 31, also lost highly regarded freshman running back Trey Sanders to a foot injury last week. Saban confirmed that Sanders also will need surgery.