It seems impossible. Galatasaray has no chance without Muslera in the games.(My opinion)
Yes, the foreign player rule creates a lot of problems for the teams, but unfortunately they have to comply with it.
Fernando Muslera is 36 years old now and he is still a really reliable goalkeeper. This is his 11th year in this team and it wouldn't be that easy for the team to sell him anytime soon. I expect him to stay at least until the end of his contract which is 2024. Rather than replacing him, Galatasaray should replace one of their useless domestic goalkeepers with a talented one. It is difficult for them to sign talented goalkeepers from their rivals of course. But they can still find qualified goalkeepers if they make a detailed research.
Until finding the right Turkish goalkeeper they had better deal with the foreign player rule by changing some other players. There are quite many foreign players in the squad after the recent transfers especially.