Mercenaries lose attack strength and speed as combat and training build fatigue. Injured mercenaries cannot deploy until they recover, and falling while exhausted can mean a longer recovery.
Open a mercenary under "Mercs" and check "Condition" in the "Growth" tab. Review the party’s deployment condition in "Tower" for exhausted mercenaries or those assigned to rest.
Assign rest and advance the week if your main party needs recovery before a boss. Upgrade Lodging to help with fatigue, and consider the Infirmary if injuries keep leaving gaps in your party.