starfish amputation?

I have a red Knob star and I beilive it was stuck somehow and I lifted it up and put it on the sand and a crab fell off of it. I flipped it over to notice the crab ate a huge chunk in the middle of one of its arm. I didn't notice it before because it didn't cut the wall just the inside. So it is alive, because it has the suction cups extended and moves but I notice flesh falling off of it and I have to pick it up and remove it. I know they regenerate there limbs so it will grow back but should I amputate the arm? Or would it do more bad then good?

Thanks!

I am willing to do anything to help him live because he is my favorite in the tank.