The server automaticly selects the players to fill the gaps.
It works like this, at the beggining of the season the players that are out of contract Will be put in the reserve spots, If you dont have 14 players with contract signed, the server Will create the number of players to complete 14 players, all of this autogenerated players Will have above 32 years old and Will retire at the end of the season.

Then to select the start eleven he Will fill the spots with the players with contract signed that are assigned in the automatic substitutions, If there arent players assigned there, he Will fill the spots starting with the player in the S1 position until he got 11 players in first eleven, except If the S1 player is a GK, the he starts with the S2 position.

If there is a player in first eleven injured he Will replaced with the player assigned in autosubs or the player in the S1 position.
But If there is a player suspended because of cards, then he Will be replaced for the player in the R1 position, the first in the reserve spot.

At least this seems to be what happens for my experience.

The server Will always try to put 11 players in the field, If that isent possible, you Will win by 3:0 and there is no game.