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Hey, can you link me the website where you got those stats for Avery Bradley?
The ACL in and of itself is only important for stabilizing knee rotation. By that I mean, with an intact ACL, you can't turn your lower leg inward in relation to your thigh. You also can't move it forward in relation to your thigh. Neither of those is terribly important in straight-line walking. Also, very strong leg muscles can sometimes give added stability to the knee, masking an ACL tear somewhat.
It also depends on the amount of swelling that occurs. There is a blood vessel that runs through the ACL, and if that breaks, as well, bleeding in the knee joint will lead to incredibly painful swelling. But if the ACL tear is only partial, it's possible that doesn't happen.
Read this article by Will Carroll, just published. It's great stuff and helps answer your questions, too.