As such there is no diet prescription in ACL.But main precautions to be taken are to avoid any strenuous activity with knee as in single leg standing on the affected knee,any jerks to the knee etc.Initial rest to the knee and strengthening of leg muscles,which further depends on the grade of your injury are the only initial physiotherapy treatment of ACL.As the strength improves you need to do lot of balance and proprioception exercises.If the above things taken care of properly 100% recovery can be expected (time taken further depends on grade of injury).