It's a thin ass piece of fabric used to stabilize the knee a bit and try to limit mobility. It becomes loose and loses it's efficiency after constant flexion and extension of the knee joint, the same way lots of other braces on various body parts do. Especially considering the fact that it probably wasn't even all that tight in the first place since your dumbass coaches thought it would be a good idea to have him try to run the ball several times in the game
This isn't some Dark Knight Rises kneebrace they have on RG3
Dude shouldn't have even been out there in the first place.