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Rust

If players go an extended period of time without playing, they will become "rusty" and suffer several in-game ratings penalties until they return to form.

The actual rust mechanics work entirely within the game simulations, so there are no visible effects elsewhere, like the roster or player screens. But if a player hasn't played for a while, you can assume he has a percentage chance roughly equal to the number of days he hasn't played in to be rusty in a game (i.e., a month-long layoff means a roughly 30% chance). If a player hasn't played at all yet in a season, the rust chance is significantly higher.

It'll require approximately two games for a typical player to get un-rusty after a long layoff.

The AI's lineup selection is aware of the rust effect and takes it into account when deciding who to dress, trying to avoid rust situations from developing.

Note that rust applies to offseason layoffs as well, so it's important to get players into preseason games to remove the rust effects.