IIRC, the book by Dettloff explicitly said back. Haven't read it in a while though tbf. Either way, GGG never fought anyone of Marshall's calibre full stop, let alone with an injury like that. Actually, he never fought anyone with an injury that bad.
An intriguing fight! Charles was of course a great fighter - and prime GGG wasn't bad either. I have a hard time picking a winner, so I have voted "Can't Decide". Though I will have to say, that the LEAST likely result is a ko win for Charles, imo.
It is highly unlikely that Golovkin would ever be able to fathom the sophistication of Charles. Much more likely that Golovkin would be mugged over the distance. Wide UD win for Charles.