All good tactics should include substitutions to ensure that a player will not be too tired to play the match.
Substitutions can happen in a few different ways. You can decide your own substitutions based on your tactics adjustment, which you can read about above.
There can also be automatic substitutions by the system before the match starts, if a player is injured or sent off and cannot play the next few matches. If this is the case the system will automatically try to find the best suited player for the position.
You assistant manager will also try to switch players around if you have a player that is either sold or retires. You will receive a notification about this, that will tell you who was switched out of a tactic and who replaced him.
You can only make 3 substitutions per match. Substituting a player that gets injured during the match will count towards the 3 substitutions.