Player condition: fitness, form, and morale
How fitness, form, and morale fold into a player's effective rating on top of position fit.
The rating you see isn't quite what plays. On top of position fit and foot, a player's effective rating on the day also folds in their condition: fitness, form, and morale.
Fitness
A tired player is a weaker one. Fitness drops with playing time and recovers with rest, so rotating a squad across a busy run of matches keeps more of it in the bank.
Form
Form is a small swing around a player's normal level, running hot or cold over a stretch of matches. It moves the effective rating a little in either direction.
Morale
Morale works the same way as form but with a smaller swing. A happy squad plays a touch above itself; an unhappy one a touch below.
None of these show up as a separate number on the lineup screen. They're already baked into the effective rating you pick from.