IME with M. doreensis, damage like that to the foot is a death sentence. I have had them come in like that about 6 times, and never had one recovery.
I wouldn't give up hope just yet, but I would keep a very close eye on it.
How long have you had the anemone? Did you look at the base of the foot before you put it in your tank?
How deep is your sandbed?
How long has your tank been set up?
What are your current water parameters? (( with numbers please ))