I never observed this. I just heal and use them as per normal and i fail to take notice.
Actually there are a few things i wanna confirm through observation.
1) I read if a players morale is on its way down, he will not play well even if we pump him up using blues.
2) Performances after healing for long term injury.
3) Carded player with falling rating. I usually sub them to prevent 2nd yellow but will try to leave them in to see if they can turn things around