Well considering i've ruptured my ACL's on both knees, several MRI's, several consultations, hours and hours of research, watched my own surgery and a combined 18 month rehab i'm in a decent position to know. Any damage to an ACL damages the integrity of it. He might also have meniscus damage which is very common with ACL injuries, but very rarely reported in ACL injury announcements. Even if it's a partial tear, every surgeon in the world would advise a sportsman to have a full reconstruction. I had a 25% partial in my right knee and they strongly advised I had a reconstruction. I work in an office and play 5 a side.
It'll be 9 months, i'd love to be wrong but I very much doubt I am unfortunately.