I'm with ya. Trust me I have had my fair share of ankle sprains with basketball. It sometimes can be worse than just breaking your ankle. I just am getting the vibe it is a mild sprain. Even with a mild sprain it hurts like hell right away and that's probably why he had to come out.
Pain wouldn't have been the reason he came out. I suspect that the muscles there pretty much shut down from the trauma and wouldn't fire -- no strength to push off to either run, pass or get to a handoff on time. Could heal quickly. Might be too weak for a while.