Board Thread:The Panopticon/@comment-25041817-20180114204444/@comment-188432-20180723215045

It's about as explicit as you can expect out of prose, really:
 * He [the Eighth Doctor] aborted the sequence, with a mixture of relief and reluctance. Of course... He already knew that the Master had hidden on Tersurus when his final regeneration had been used up. Some devastating force had ravaged his body and left him a crippled wreck.


 * But his TARDIS had left the world.


 * That could only mean that Susan had been the one to trigger the Master’s grotesque change. And that she had taken his TARDIS and gone on alone. There was no need for him to go to her aid, then. She had acted swiftly and certainly, and solved the last remaining problem.

So:
 * Delgado is the last regeneration (of what we would later discover in Utopia to be the original lot of regenerations).
 * Whatever caused him to look like Pratt/Beavers was not regeneration but a "grotesque change" and a "devastating force".
 * Susan's the one who did it. And she stole the Master's TARDIS in the bargain.

That's just not reconcilable with the comic explanation depicted vividly in Doorway to Hell.