Since the knee had a direct impact, and fracture was ruled out , the possibilities are
of the knee
2) ACL tear ( since the force was from ant to post.)
tear is unlikely since there was no twisting injury
2) Immobilization ( knee brace
3) Ice pack
4) After 5 days ,once swelling comes down, repeat clinical assessment to check for ACL.If strongly suspected, MRI SCAN is the plan.