For the newbies, player fatigue is something that needs to be addressed to be avoided. It is common for it to happen if one is unaware of the problem. It most generally happens with pitchers in the minors but can happen at the ML level to the unknowing.
Pitchers have two stats that tie directly to fatigue, durability and stamina. Stamina determines how many pitches one can throw in an outing. For a starter it is common to use the stamina rating plus 15 for the TPC then add about 15 or 20 for the MPC. Therefore if a starting pitcher has an 85 stamina, he could throw 100 to 120 pitches an outing. Now for the kicker, if the pitcher is in a 5 man rotation this should be fine. However, his rate of recovery is based on his Durability. If he has a low durability, say 20 or less, then he may not be ready for his next start fully rested. Above 25 shouldn't be a problem.
Many starters however, do not have the big stamina like those in the upper 50's and 60's. Most of the time these pitchers are middle relievers but can be used as starters as well. They normally only go 5 and maybe 6 innings a start. There are few complete games so a good pen is necessary.
Short relievers normally have a 30 stamina or less but usually have a high durability.
Basically if a pitcher gets fatigued he can't pitch well or not at all and that causes problems. In the minors a team can have spare pitchers on the inactive rosters to compensate for fatigue.
Most teams carry 13 pitchers on their roster usually consisting of 5 starters, 2 long relievers which can also be starters, 3 middle relievers and 3 short relievers, one being a closer.
Another thing that can hurt a pitching staff is extra inning games, so one must be careful to keep them rested.
Position players can also become fatigued but is not quite as serious in most cases. A position player becomes fatigued when his durability exceeds his number of plate appearances to the end of the season. A mouthful I know. Basically, you take 730 * .durability = plate appearances for a season. Players with 85 plus durability will probably play the entire season without fatigue being an issue. Those in the upper 70's however will need a day of rest here and there. Lower 70's will need more. Notice I said Plate Appearances and not AB's. PA's can be found under World Office-Stats/Awards-Player Statistics. You will need to choose Extended in the split box.
The reason fatigue is of importance is players can become injured or useless. Pitchers that have a 0% fatigue rating could take days before they are ready again.
For Shafty: If you were in a division where all your rivals weren't going to win 90 games or more I would have been a bit more generous. So I will help you out.