Players do never retire in Futuball.

When a player is born he has a small number below his name on his player card. This is his "energy".

Energy is amount of match time left to play before reaching critical condition. It is typically around 250 competitive matches a player can play from being created.

Friendly matches, world tour matches and ladder matches does not take much energy from a player btw.

When a player reaches 0 energy, his condition will change to "critical". At critical condition a players attributes will drop to 1/4 of what they are normally.

You can still play a player when his condition is critical, but he will not perform very well because his attributes have dropped a lot.

The only way to get more energy for a player is be recharing him in the cryo chargers. But as there is only room for 3 players to be recharged at the time, it might be better to dismantle and sell your player for a few F-coins and instead rebuild a new player on the empty bot you get back when you dismantle your player.