It's much more than that. Casillas is no longer a viable top level keeper. He has been booed a few times. Navas for all his heroics will never be more than second choice anywhere.
Plus with barca sweeping the titles and looking stronger than ever (even reportedly adding pogba whom Madrid would have wanted) have put Perez under enormous pressure to bring about changes. De gea would have probably appeased the fans a little bit as he is future Spain no.1 and arguably the best keeper that they can get. If they wait one more year they cannot survive the barca onslaught with the existing keepers. They are more desperate than we think