It vanishes after a week, when the new training updates happen.
Still an18 year old won't lose skills, exept for stamina.
If your players seem to play worse, that might be, because of the game shape.
Your Center Ames for instance played for about 120 minutes last week (all three games combined).
Your player should have like 60 minutes, so it has a good effect on his game shape.
120 minutes is way too much, so the game shape gets worse.