he doesn't know it but you're doing what is best for him right now. when he is stable (medically wise not balance wise) then you can put him back but still keep an eye on him.
he could have thought he was a rodent, i've found mice in my duck inclosure that are crushed and bloody. you might consider putting some hardware cloth or 1/2'' chicken wire around the inclosure so the ducklings can't get through
i think he was traumatized!
take the other ducklings out of there in case it happens again.
make sure he is eating and drinking!
and when he is awake see if he can walk