I think "degenerative" refers to the knee cartilledge (meniscus) wearing away more and more - you get bone on bone grinding and constant pain/swelling - really limiting strength and mobility. Basically (as you get older) this turns into osteo-arthritis (I have this, which is why I know - it came from 2 ACL tears as a late-teen - and then wear and tear on "degenerative" knees).
The oline FA we signed just after the last draft probably has same sort of thing - you can play on it for awhile but it slowly (and sometimes quickly) gets worse and worse, and you just aren't the same (speed, strength etc).