In several areas, such as the GM's Office, the players are displayed with an "ENG" or "Energy" percentage. Each player's energy can range from 0% to 100%, where 100% is no fatigue and 0% is completely exhausted.
If a player is fatigued you will want to rest him for a few games. The amount of rest needed depends on the player and what condition he is in.
A player's condition is based on how much they play in this SimLeague compared to how much they played in real life. If you have a player who only played 25 games in real life and you are playing him every game, he will fatigue a lot quicker than a player who played 80 games in real life. This also plays a factor in recovery time. The player who played 80 games will probably fully recover to 100% after resting for one game. Depending on how fatigued the guy who played 25 games is, it may take him 2-4 games of rest before he recovers to 100%. A good idea to minimize fatigue is to make sure a player is not playing on too many lines and pairs. In some cases, just reducing the number of lines or pairs he is on may help a player recover to 100%. Remember, fatigued is based on time on ice, not games played.
To rest a fatigued player, simply make sure he is not currently set to any lines or pairs. An easy way to verify this is by clicking the "View All Lines" and "View All Pairs" links found in the Coach's Office in the Line Management and Pair Management boxes. The only way a player could play when he's not on any lines or pairs would be if your "Sim Coach" decided to put him in to replace an injured player or if there was a line/pair conflict.