While I don't expect pro-level teams to divulge what's going on with all their athletes, let's be clear that there's a significant difference between a strain and a sprain.
From the Mayo Clinic website:
Sprain - A sprain is a stretching or tearing of ligaments. Ligaments are tough bands of fibrous tissue that connect one bone to another. Common locations for sprains are your ankles and knees.
Strain - A strain is a stretching or tearing of muscle or tendon. People commonly call strains "pulled" muscles. Hamstring and back injuries are among the most common strains.
Now I don't know about you, but when I see someone's knee buckle like that, it's pretty rare that it's a simple "strain." The exception would be if someone blew their quad tendon, which he doesn't appear to have done.
Instead, when buckling occurs it typically occurs due to ligament damage - possibly a torn ACL, MCL, etc. I'm no sports medicine doc, but I'd be surprised if this was a simple muscle pull and that he's out a week or so. (For his sake, however, I hope I'm wrong - he's a helluva player.)
So as I stated up front I don't expect teams to divulge all their insider secrets, but at the very least the reporting of injuries leaves a lot to be desired for an anatomy geek such as myself.