If a player has been injured or suspended before a match, or if he is injured during the match, then the system will attempt to put in a reserve in his stead.
The replacement player will be found in the row of reserves corresponding to the pitch placement of the injured or suspended player. See the section named Reserves for information on which pitch placements correspond to which row of reserves.
If you have not set any reserves corresponding to the placement of the injured or suspended player, then no substitutions will be performed. It is therefore very important to make sure that you always have reserves set up.
Remember that you can only perform 3 substitutions per match. Substitution of a player who is injured during the game also counts towards this limit.