I think it is worth pointing out that Diaby has the same injury as Walcott has, has had the same surgery Walcott will have, and Wenger today is doubtful whether he will be back this season at all.
I know Diaby is a special case when it comes to injuries, but I think it is fair to point out that when talking about ACL injuries it is 6 months minimum. I'd even go as far as saying that 6 months is very, very optimistic.