I watched the hit a couple of times trying to figure out how the injury happened. It looks like the main point of the hit was just a fraction above the knee with some of it hitting the patella itself and pushing it around. It looks an awful lot like this very unlucky spot caused his patella to pop out. A more direct hit to the knee/patella probably wouldn't have been nearly as damaging in most circumstances but tearing away/dislocating the patella is far, far, more damaging in most cases. If you check your own kneecap/patella you'll be able to see that pressing from just above it (or to the sides) will wiggle it around, especially when the leg is straight.
The problem with this injury is that there are few, if any, ways of knocking the patella completely loose (like it looked like on video) without doing both cartilage and tendon damage.
This injury had already depressed me to see on the non-HD stream I watched as it happened but seeing it in HD, it looks so much worse. I really, really, hope and pray Marcus can return from this and that it looks worse than it is -- maybe I saw something else and mistook it. But I'm very worried for him.
Marcus his family, his friends, and his team are in my thoughts. This injury has given me flashbacks to Inky (another great guy) though I don't think Marcus will be crippled. It seems so unfair that stuff like this happens to so many good people.
I'm heartsick.
All the best Marcus. Our thoughts and prayers are behind you.