it depends was the operation how well was it , did the tissue heal well ?
but then it could be PFPS --> patella femoral pain syndrome , which mostly comes from weakness of the quadriceps muscle specific the VMO this is the main muscle that becomes dysfucntioned after injury and it is on the medial side of the knee which causes the patella and all the knee to be less stable(average jo will not have issues but a professional athlete will) so now from light training to a few games in a row (with hte young team and now serial a) to full force crazy speed and intensity after such long time the body didn't experience it and that tiny lack of stability caused pain in the knee (which is nothing connected to ACL , but more to the muscle imbalance and long rest) so knee need to be reseted , muscles need to adapt, could take another 8-9 month for muscles tendon to comeback to pre injury strength , and not 9 month from the operation , but 9 month since he went to full force training... the brake of the world cup is exactly what he needs...