Baseball players suffer from two types of fatigue: day-to-day (short-term) fatigue, and seasonal (long-term) fatigue.
Rested > Good > OK > Tired > Fatigued > Exhausted
Performance penalties, in the form of reduced effective ratings, begin when a player is Tired and gradually increase until they are Exhausted. Players will not start, and pitchers will be removed from a game (if possible) when Exhausted.
All skills are subjected to performance penalties with the exception of EN and SY.
Pitchers fatigue based on pitch count, at a rate of about one point of Endurance for every two pitches. So, a pitcher with a 50 Endurance can throw 100 pitches before becoming Exhausted and taken out of the game (when possible).
Each player has a current and maximum Endurance - when a player is fully rested these values will be the same, ex. 60/60. All players lose current Endurance points when playing; players with higher Endurance will fatigue proportionally more slowly than players with lower overall Endurance.
Player A: EN = 80/80 = 100% (fully rested)
Player B: EN = 40/40 = 100% (fully rested)
Both play a game and tire similar amounts
Player A: 78/80 = 97.5% rested
Player B: 38/40 = 95% rested
Season fatigue is the wear-and-tear a long season can have on a player. Season fatigue has two impacts:
If a player, doesn't regain all his Endurance as expected, you can bet that he's suffering from season fatigue in some degree.
Note: Each game at DH causes less potential season fatigue than when playing a position. Toggle your use of designated hitters to extend low-endurance players.
A player will recover current Endurance at a rate of about 1/4 their maximum Endurance. That recovery rate gradually gets smaller as players accumulate Season Fatigue.
There are a few ways keep your players rested, aside from manually adjusting lineups between games.