I actually had one that did that very same thing (except it was a turkey)... hatched by itself without absorbing all the yolk. I was terrified that he'd tear it, as he would not be still and was all over the bator like a wild animal being pursued! He was the last of my poults to hatch so I moved all the other poults to the brooder and here's what I did.
Now, I'm not advising anyone else to do this (so don't beat me up!), but will share my experience. I got my first aid kit out, found a roll of 2 inch gauze, cut a strip about 6 inches long, folded it in half (now it is 1 inch wide) and fashioned a "diaper" with it, fastening it with medical tape. It was not snug, but fit well enough that the poult couldn't crawl out of it. This completely covered and protected the area where the umbilical cord and yolk was and prevented it from being torn on the wire bottom of the bator.
After convincing myself that I had done everything I could, I placed the poult back in the bator and called it a night. In the morning, I found the little guy still wearing his diaper and screaming to be let out of that bator. I took his "diaper" off and he appeared to be fine. Now, I will add that he sorta has an "outie" which I honestly believe will slowly disappear. It reminds me of a newborn's umbilical cord stub which has dried up but not fallen off yet. It is actually turning black and appears to be in the process of doing just that. He's a very healthy, energetic poult and acts no differently than any other of my poults.
I will add... had this little poult not been so energetic, I would've just wrapped him in a warm, damp paper towel, as suggested before and let him be.