By now they'd already know from a preliminary scan had it been an ACL rupture, so I'm thinking that can be ruled-out. Strain, sprain, or minor cartilage damage, he could be back in 2-3 weeks. Ligament tear or more significant cartilage damage would be grimmer news.
Won't be able to differentiate among these possibilities until a re-scan after his swelling goes down. Having experienced all of the above, I know that initial pain response isn't useful information... In fact I had no pain at all, even when walking, when mine (admittedly already damaged) ruptured, though I did feel a pop and knew that it was not gonna be good.
Given Kun's outstanding physical condition, he has a better chance than most of it being a 2-weeker. Will be anxiously waiting the next couple of days with fervent prayers for Kun...