The premise is that Dr Solon has created a body, much like Frankenstein's Monster, made up from a variety of alien parts, to house the still living brain of the evil Time Lord Morbius. To complete the body, he only needs the Doctor's head. But instead of hacking the Doctor's head off, why not use the Doctor's entire body and simply transplant Morbius' brain into it? There is no logic in making Morbius live in such a horrific body when something more like his original form could be used.