Characters, including The Warden, that fall in combat receive a debilitating injury that lowers an attribute until treated. These injuries can stack. To remove a persistent injury, you must return to your party camp, have your Spirit Healer use Cleansing Aura (be aware that Cleansing Aura will not remove an injury on the Spirit Healer themself), or use an injury kit.
As with poultices, each kit has a certain level of potency that determines how much damage it's able to repair. Lesser injury kits heal a single injury and a small amount of health, for example.
Possible Injuries
Injuries in Dragon Age II
There is only one injury type and the result of an injury is temporarily reduced health until the injury is removed. Injuries can be removed with a potion or injury kit by entering the party selection screen and confirming. There is no need to actually change party members.
The maximum number of injuries one character can suffer at the same time depends on the difficulty level.