I'd argue we can replace anybody in the back-three expect for David Alaba, even Jérome Boateng with his exceptional laser-like and precise build-up. I don't know if even many Bayern fans realize this, but right now Guardiola and him are inventing a new position nobody has ever played quite like Alaba in the history of world football - or at least in the past decade.
A player who's capable to play as a LCB, who is strong and quick, can pass the ball, is an excellent box-to-box midfielder in his own right and defines all of his team's transitions from attack to defence and vice-versa. That's extraordinary in many ways, but especially because he's not getting exposed in defence.
I have been wondering why David Alaba has not been growing as a pure LB last season. His performances seemd a little underwhelming overall, but this season shows that he's probably been eagerly anticipating an opportunity to prove his worth in a more central role and he's simply extraordinary.