If a low grade infection is found in the joint fluid after acl repair is there any risk to the graft itself or problems in the future?

Yes: The "low grade" infection can infect the graft, the joint (destroy the cartilage) and or invade the bone and give you a bone infection in the bone tunnels. Antibiotics are a must as well as serious consideration to irrigating the the knee out arthroscopically 1-3 times and if it progresses removing the graft itself. F/u closely with your surgeon