My gut reaction upon seeing Willis's injury today was a ligament tear in the knee, specifically an ACL, which frequently results from a hyper-extended knee. While some will take the fact that he continued to pitch as a fact that the injury is not serious, we simply will not know until an MRI is performed. You can still walk and potentially throw balls to the backstop with torn ligaments in the knee. The ligaments aren't there for support, they are there for stability.
Here are a couple of reads on hyperextended knees, one from my favorite new blog, the Disabled List Informer.