Are there really major arteries at the back? He got slashed in the waist. If it's possible, I'd rather he get slashed than stabbed.
@ Demi - Compare this:
http://danbooru.donmai.us/post/show/...isole-flower-g
Please be aware that that's a guy.
Yes, that would be very bad. Unless you plan to replace it with prosthesis, I suggest you don't do it.