If your Companion takes some damage during combat, they need time to heal afterwards. During the time they are healing, they cannot be taken on another mission.

The amount of time they’ll take to heal depends on your player level, the mission type, and the amount of damage they've taken.

If your Companion is injured, they take a short time to heal, and if they collapse to the ground, they will take longer. After getting injured in an Infestation, they’ll need a bit more time to heal than if they were injured in an Encounter.

If you want to get them back up and going for another mission you can quickly heal Heroes with a First Aid Kit. It's a green icon with a white cross.