I agree w/ Rhyme. Schools say that they require an explanation if you don't have a current supervisor write a rec, but this doesn't mean it's a bad thing. You can just say something like, "I would like to postpone notifying my manager of my intentions to attend business school until I have made a final decision..." or something like that.
That sucks about the situation with the guy and the back surgery. Did you guys write rough drafts for your recommenders? If you're going to give up on the back surgery rec and go with your current supervisor you should make his/her life easy and just write a rough draft and say to them, "Here's a letter I put together with some of my accomplishments, I thought this letter might give you an idea of the structure that the school's are looking for, feel free to use as much or as little as you would like."
This will guarantee that you get a quick turnaround on your rec.