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@okcjim I'm only a med student, there are people who are more qualified to comment here.
1. It's not about his ACL (anterior cruciate ligament). It's about one or both of the menisci in his knee. Wiki it. It consits of cartilage and reduces errosion between the femur and the tibia. It's a very important part of the knee joint.
2. Cartilage regenerates very very very slowly if at all. By looking at it with different imaging you can say how far it degenerated already.
Degeneration of the meniscus leads to arthritis in the knee thus limiting the range of motion.
Again google it. The internet knows more than I do :D
2 years ago on NBA Scout: Perry Jones’ knees will only hold up ’3-4 years’