As Dan Fouts would have put it, "That is a serious knee injury for Ryan Damn Shazier"
We just learned about this in school. It can be a really bad injury but obviously depends how bad the tear is. The MCL is actually connected to one of your meniscuses in your knee and almost always tears that too. Plus it causes the knee to buckle, usually tearing the ACL. They call it the "unhappy triad". It's really common for football players, apparently esp. at Purdue.
Dude was a monster for playing with this. He had to be in a crazy amount of pain. And he would have been even more of a legend if we actually won the game with him on one leg. But unless it only a small partial tear, he was playing with fire by not coming out. Either way Shazier is a MAN and I'm currently drawing pictures of him in my notebook with hearts around him and practicing writing 'Damn-Shazier' as my last name...(swoon).