good news it's looking like. Reading between the lines now of the trainers and others comments, it's apparently not structural meaning any sort of significant tear of ligaments or cartilage (aka meniscus'). So it's either what Marsh posted the other day as a grade one sprain to one of the cruciates, likely mild PCL as he said he came down on it wrong and someone said they saw it hyperextend, but a very mild grade one likely more stretched vs. torn. The other possibility could be he sprained the outer capsule which may go along with a mild bone bruise of the other type cartilage that lines bones, this sometimes in combo with the other mild injury.
Regardless though the good news is that with any of the above he should be full go in 4-6 weeks with Rx!(and if they don't rush him, Pleeeeaaaase don't rush him!)