He only had 2 significant years at USC. He was overshadowed by other great Lbers, and had to wait his turn. The disease I think really hurt him, he had trouble maintaining weight let alone adding weight. He's undersized by NFL standards 6-0, 226 pounds is really light for an NFL LBer.
So I could see why he wasn't taken high. There are alot of great raw athletes that never develop into great NFL players. As PC said in his presser on WED this is the BEST he's played in the entire time that PC has known him.