Whenever a player is booked (one yellow card) he receives three disciplinary points, and for a sending off he gets five. Once a player reaches 20 points, he will be suspended for one match. Every ten points after this he will be suspended for one match. Sendings off (two yellow or one red card) will result in an automatic two match ban. Your Squad list will show any players who are unavailable due to suspensions.
Although it is not very realistic, because you have already chosen your team selections, players who are injured will still play in any remaining matches that week and only become unavailable the following week. Your Squad list will show who is injured and for how long. Injuries are usually for just a small number of weeks.
If as a result of injuries or suspensions, a team can not field eleven fit players, untested youth players will be drafted in. These will enable you to play your match but they will have low skill factors. It is best to make sure you have a large enough squad to provide cover.