Spoilers: The Christmas Invasion, Journeyâs End.
Summary: When youâre as brilliant as the Doctor, you can find a way around almost anything.
He doesn’t want to regenerate; he likes this body, feels like he’s barely got used to it, but even a Time Lord can’t simply shrug off the effects of a Dalek ray. His body’s damaged too severely not to regenerate, yet he fights it. He needs to heal, but maybe he doesn’t need to go through a full regeneration, not when he’s got a convenient spare part just lying around not doing anything.
As the golden energy fills him to the brim, he absorbs what he needs and transfers the excess to the hand he lost fighting the Sycorax.