I'll try to explain my personal experience with this as well what's presented at various clinics I've attended. Strength has to be built back up in the knee which takes time, and it's different for every person. Lots of tedious stregthening excerises with bands and body weight, then light jogging, then half speed, 3/4 speed... you get the idea. Straight line movements are easiest, the lateral movements are the ones that take time and confidence. The knee is "sound" in 6-8 months, then the athlete has to rebuild the lost muscle strength in the leg and gain their confidence back. It's difficult to explain, but they just don't go "all out", or least the 6 kids I've worked with haven't. It takes a long time for them to get confidence back and just play, so everything they do is just plain slow until they get it in their head that the knee won't be a problem. It's difficult to explain, there's just no burst in their movements. Typically a player can be back playing in games in 9-10 months, but it usually takes much longer for them to get back to full speed, and that depends entirely on the individual, as that part is entirely mental. On the mental side the extreme case I was involved with personally was a girl basketball player. She was averaging double digit points and rebounds before the injury and a great defender for the team as a sophomore. She blew out her knee in early December and by April she was rehabbing every morning on her own (impressive dedication I might add) without any assistance, light jogging and stregthening phase. I kept an eye on her while we did plyos and weightlifting with the other kids, but she did a fantastic job sticking to her program. We just expected with her dedication and work ethic she'd make it back in record in time, and she did. The problem was that she was so slow it looked like she was moving in quicksand. We thought she'd get better as the year progressed but she didn't, and she was the exact same player a year later as a senior. Her stats basically were cut in half the rest of her career, and she couldn't guard an 80 year old man in a wheelchair, she just wouldn't push the knee at all. I'm sure Yo won't be that timid, but if he starts compensating for a lack of confidence in his knee and it throws off his delivery... well now he's looking at problems in the other joints in the same leg. Sometimes when an athlete favors one leg they create an issue in the other leg, with a pitcher maybe an issue with arm if he starts trying to over throw if he isn't generating enough push initially. It's best in my opinion to just him progress at his own natural pace, and not help him create any further issues by rushing him.