It's not an easy decision for Stegen. He is not playing if he stays. Not without some major voodoo.
So it's down to either:-
1) leave now, don't have the surgery and risk not being at the level for the world cup by performing injured, and hurt for a season.
2) stay, get surgery and seek a loan in January, where you will need recovery time and may not even be at the level to play second half season.
No club will sign him on loan if he is crocked. Only way he exits needing surgery is on a transfer, as an opportunity. And for that we run into the issues of deferral payments and what kind of wage a buyer would be willing to give to a guy who may be finished.