It depends. You need to get ahold of all your medical records that has anything to do about your ACL surgery and any follow up reports pertaining to your knee after the fact, ie any doctor notes saying that it is normal.
Then it is all up to the MEPS doctors. If they feel that your knee will be an issue then they may deny you. They will very likely want to have their own doctors look at it as well to make their assesment.
Overall, just have your documents ready and take them in with you to the recruiting office. Keep a positve outlook but know that nothing is 100%. Good luck.