The health center keeps your players playing. It decreases the chance on injuries during a match and also decreases the length of injuries if they do happen. It also causes your players to recover faster from fatigue.
As the training complex increases the injury chance, it's important not to ignore your health center when upgrading your training complex.
Once the health center is at least level 3, you can pay money to speed up their healing process. The higher the level of your health center, the more effective this feature becomes.
The amount of money you have to pay to heal your players depends on the time a player is still injured and on the weekly wage of the player. You also need a credit for each time you heal one or more players.