I was rolling with my largest training partner (230 lbs) and shoot a double. He sprawls on me really hard, and we turn into a wierd angle where all of his weight was on my leg which was bent and turned on the ground (felt exactly like a heel hook) I heard something pop a few times near my thigh and we stopped right there. I stand up and notice that I can still walk, and nothing noticable was damaged. We continue with me on bottom of guard. He reaches behind with both arms to attempt an unorthodox guard pass, and when my left leg bent a little to the left, the pain was unbearable on my left knee. He said that he recognized this injury, and I sprained my MCL (???) on my left leg. Really sucks... but hopefully it will heal quickly.