I'm OK with one knee injury. It happens. Young guys recover.
Multiple knee injuries, like multiple foot injuries ... that just doesn't bode well.
At least with broken bones e.g. Embiid, you can expect it to heal and then maybe the guy is still growing and he becomes solid and durable.
Knee injuries that require surgery to repair torn cartilage ... that's different. Cartilage doesn't heal the way bone does. I don't like the idea of drafting a guy with bad knees.