Substitutions:

Substitutions provide a safety against players who do not play in a match. The computer will automatically substitute in a player with the next highest substitution priority while maintaining a valid formation.

For example, if your starting goalkeeper plays 0 minutes, your backup goalkeeper will be automatically subbed in. If any midfielder plays 0 minutes, the next highest priority player that maintains a valid formation will be automatically subbed in.

You may change your substitution priority by swiping down on your Team page. This will reveal your subs. You may edit your substitution priority here by moving players into the order you desire.