Fortunately, you won't have to worry about that. Injustice 2 has an auto-save system that does the work for you. The game will automatically save your progression at the end of each completed match.

How do I save my injustice account?

If you are signed into a Google+ account, you can enable "Back up my data" via the "Backup & Restore" section of your Android device's settings menu. Once cloud saving has been enabled, your game will automatically save to the Google Play cloud after completing a match if you are connected to the internet.

How do you save in injustice 2?

Cloud Saving: How do I backup or retrieve my Injustice 2 mobile game save? You can back-up your game save to the cloud if you are playing using either iOS Game Center or Android Google Play. You can view the Settings menu in-game to confirm that you are logged into Game Center or Google Play.