Watched it about 10 times in slow motion.
The Illinois player stepped on the inner part of Eugene's foot as he ran by, which looked to be incidental. As he had his weight on Eugene's foot, Eugene tried to shift left to guard his man - at which point, his leg torqued with the foot being anchored to the floor.
I've dislocated my knees (both of them) several times, and I've had this exact thing happen. I was trying to pivot in cleats, and the cleats stuck into the ground and my leg torqued and the kneecap slid out of place.
First time it happened to me was in high school, and it was about a 6 week recovery, if I remember. Swelled up like a grapefruit a couple hours after the injury. I had some partial ligament tears, but didn't need surgery. Overall it was ice/elevation, crutches and a leg immobilizer - then physical therapy.
Really hoping that he didn't do any significant damage in there.