Yeah, it's probably a severe sprain. Which is like, real bad. Rondo might be out another week or two. Hard for this team to win any games without Rondo.
As I've said before, this team was going to struggle to win much more than half its games without any major injuries. If Rondo misses more than a few games, we might as well just start sitting the Big 3 and just all-out tanking.