Indeed, losing Ter Stegen is one of the problems faced by Barcelona, especially since Ter Stegen's injury is quite serious. But I believe Barcelona's performance will not decline even though they lose their main goalkeeper because Hansi Flick can handle it, especially since this season Barcelona has lost many of its key players but Barcelona's performance is still perfect where Barcelona has not been defeated in La Liga until now.
Ter Stegen's injury is not the first that Hansi Flick has faced since he took charge of Barcelona, previously several important players had also entered the treatment room, but he always managed to get through the difficulties in his own way. Now Flick has turned his attention back to young players from the La Masia academy (Áron Yaakobishvili), he has started training with Iñaki Peña and Ander Astralaga, all three will be included in the squad when Barcelona host Getafe at Camp Nou.
Martinez is under contract with Aston Villa so Barcelona will be unable to gets him because transfer window have already closed Barcelona is only can gets the players which free transfer status and according to Barcelona medical team André ter Stegen will be rested approximately for next 7-9 month ~Snip~
Barcelona are also considering two options to recruit a new goalkeeper to replace Ter Stegen.
1. Recruit a Free agent goalkeeper (now).
2. wait until the January transfer window opens to bring in a better goalkeeper (as a long-term option).
But it all depends on the coach.