There are Stamina Ratings for passing, receiving, and rushing. The higher the ratings, the better. These Stamina Ratings determine how much the player can play. For instance, the Pass Stamina Rating determines how many passes a QB can pass in a game, while the Rush Stamina Rating determines how many carries a player can make. If a player is playing more than his Stamina Rating allows, then his performance will begin to drop. This means it is important to draft a team with enough stamina in each category to get you through the season.
Other than the Stamina Ratings, the other thing that affects performance during the game is fatigue. Fatigue only occurs within each game and is accumulated as the player plays. The bigger the play, the more fatigued he will be. Fatigue is recovered by sitting out some plays. Fatigue is not accumulated over the season, so if a player is fatigued in one game, he will be refreshed for the next game.