From the Rule book. Note the last paragraph.


Rustiness: Rustiness results from players not appearing in a particular number of consecutive game while on the major league roster. That number varies according to the role of the player. Regular players become rusty faster than part time players for example. Players who are returning from the farm team (inactive roster)are not considered rusty.

Players sent to the minors never come back to the majors "rusty", even if they are "rusty" when they are sent down. One exception to this rule is after the regular season. During the play-offs, if a player is sent to the minors between play-off series, he can return "rusty".

NOTE: The game does not enforce rustiness on ROAD players or when the ROBOT is managing the home team. Rustiness is only enforced on the HOME team players when the game is played by the HOME manager.