There is a modifier in Football Manager that affects how soon your players get jaded, which is determined by how demanding your preseason is.
According to Rashidi (Bustthenet) it's recommended to push your team to the hilt in preseason. Why?
There is a modifier in the game that affects how soon your players get jaded, which is determined by how demanding your preseason is.
Rashidi showcased the effects on a livestream.
In one season he left it on default, played 8 games, a few of his players started getting jaded by January, while their performances suffered too.
The next season he did bootcamp for 3 weeks and had 7 friendlies. Only one player was jaded, and that was in the last game of the season.
As you can tell, a hard pre-season helps the team maintain fitness throughout the season.
So make sure to push your players to the limit with their preseason training because it's crucial to avoiding jadedness and poor performances later on.
Source: community.sigames.com