The 20-year-old will require surgery after tearing the anterior cruciate ligament in his left knee during Tuesday night's defeat by Wolves.
Afobe made five appearances for Millwall after making the switch to south London last month on a deal until the end of the season.
A Lions statement read: "Everybody at Millwall FC would like to wish Benik a speedy recovery and look forward to seeing him back in action as soon as possible."
Millwall boss Kenny Jackett this week also lost winger James Henry for the rest of the season due to a knee injury.
- Sports & Recreation