From the Clippers' press release:
The MRI revealed that Livingston suffered tears to the anterior cruciate ligament (ACL), posterior cruciate ligament (PCL), medial collateral ligament (MCL) and lateral meniscus. Livingston also suffered a patella dislocation, in addition to the previously diagnosed tibia/femoral dislocation.
I'm not a doctor, but I'm pretty sure that's not good.
In laymen's terms, his knee exploded. The three main structures holding the knee together are torn. Of the four main ligaments in the knee, only the lateral collateral ligament (LCL) was spared. ACL, PCL and MCL - that's the trifecta my friends. To put it into perspective, a tear to any one of those ligaments is career-threatening. He tore all three. I've literally never heard of such a thing.