User blog comment:KillRoy231/Why'd they have to spare Gregor?/@comment-5014364-20150512031646/@comment-124.6.181.204-20150601212123

And maybe it wasn't the Mountain's head. In the book is just a skull, all the flesh removed so as to not spoil in traveling from Kings Landing to Dorne. Only the huge size of the skull shows resemblance to the Mountain - he may have his original head.

Also, when Cercei in the book is finally allowed to return to the palace, after a humiliating naked walk thru the town, she is told that her 'super champion' is ready to fight for her in the by now familiar justice-by- dueling-champions (as happened twice with Tyrion), and it is strongly hinted that Cercei's champion will be the re-animated Mountain.