Each team only allows 3 substitution for each game. If a player receives two cumulate yellow cards, that player will get suspend for one game. If a red is receives, that player will get suspend by two games. Training the players can make their skill improve in most of the time, but it also has a chance for a player to get injure. So sometimes it might be a good idea to hire an extra player or two in case of suspension or injury.
Fitness can help players with small injury to recover faster so they can rejoin the team earlier.
Although the more expansive players are always better than the cheaper one, but remember player also have to think about the finance of the team also. Make sure to check out the finance of the team every once awhile. If the soccer club runs out of money, the game will be over. So do not think hire all the best players in the team will lead to the victory because it might just drain all the money out of the club so fast. Sometimes draft youth players might not be such a bad idea also.