Bottom line is that he's not getting adequate blood flow to the foot to keep the tissue alive. They mention in the article that his doctors are vascular surgeons. They are probably trying to figure out if they can either open up his current arteries or bypass the blockages with some sort of graft. The problem is that your arteries get smaller the farther they are from the heart and those in the foot are about 2 mm. Once problems begin, it's tough to keep them open without shutting down again.
I pray he keeps his foot. However, if he loses it and ends up in a prosthetic limb, I hope he is open and proud without an ounce of shame. Matrob is right: people thrive with them.
I used to be a scout leader and I once asked two brothers to come speak to the boys. They had lost their legs in a car accident and both of them were fit, energetic dudes who competed and won medals at the para-olympics. I did this because there when I was about ten, there was a very nice lady in my neighborhood who was missing a leg. I had to collect money from her each month on my paper route and she scared me, for no reason other than her missing leg. I brought those two para-olympians in front of the scouts and celebrated how bad-ass they are because I wanted to rid the boys of that stigma from a young age.
If Coach loses his foot, he can teach the same lesson on a massive scale.