Wikipedia wrote:A partially torn ACL will usually be allowed to heal itself. A completely torn ACL will not grow back, probably because the lack of blood supply near the ACL. It must be replaced by surgery or left unattached. The ACL primarily serves to stabilize the knee in an extended position and when surrounding muscles are relaxed, so if the muscles are strong, many people can function without it. The term for non-surgical treatment for ACL rupture is "conservative management", and it often includes physical therapy and use of a knee brace. Lack of an ACL generally increases the risk of other knee injuries such as torn meniscus, so sports with cutting and twisting motions are not recommended. For patients who frequently participate in such sports, surgery is often recommended.
So, he should be able to function without it, provided he doesn't rip any others while he's chopping up the field for the rest of the season.