Sorry to hear about your injury. I tore my ACL in June, but didn't have the replacement surgery until December. I had a patellar autograft done which I am very happy with. My knee feels more stable and I am able to bend it without being in pain more so 2 months post surgery than I was able to 5 months after my injury but before surgery. My Surgeon said that I shouldn't return to Aikido for at least a year, which I am ok with.
However, as Janet said, there are people out there who are training without acl's who haven't had problems. I would guess the reason he suggested you have replacement surgery is because you are at a higher risk for meniscus damage/tears which can be very painful and cause instability. You are also at a higher risk for arthritis. Whether you have surgery or not you will still need to keep those knee muscles strong.
Just a few things to add to Janets post- If you do decide to have surgery make sure your doctor does ACL's and has a good record for success. make sure you feel comfortable asking him questions and that he takes the time to answer them. Ask him about pre surgery physical therapy. It might be frustrating to work hard to build your knee strength up only to have surgery and have to start all over again, but it will make your recovery smoother and hopefully quicker.
Follow thier instructions- allow your body time to heal, don't push yourself faster than what they say because it may result in problems down the road, however if you do have surgery you are in for a long rehab process so stick with it. It will pay off.
If you have any questions feel free to email me at firstname.lastname@example.org
. You can also check out my weekly acl diary at http://home.comcast.net/~mouse7000
It might give you a general idea about recovery times. I have learned to appreciate the small "baby" steps. Obviously I havent returned to Aikido yet, but i plan on it and am working towards it. Right now I am able to hold a few of the stances, lunges etc for 30 seconds-1 minute but no twisting or turning yet.