First Aid?

We had a pair of blackbirds living in our back garden and the male used to sing beautifully.  Sadly, yesterday morning my husband found it unconscious on its back after being mauled by a cat.  He put it on a planter in a quiet spot at the side of the house to recover and see if it would fly away again, turning it the right way up before leaving for work.  I checked on it about 45 minutes later, and it was perched on the planter but hadn’t flown away.  It’s back had a hunched appearance as if there was something wrong with it.  It didn’t occur to me until later that it could be a perforated air sac that was shown in a recent RSPB magazine.  In the meantime, I rang our vet and took it there to be put to sleep.  I wondered if I could have done more to help it, e.g. offer food and water.  Does anyone know how long a bird can take to fly again after this injury?  I didn’t want it hanging around as the cat would come back to finish the job and I thought it best if it couldn’t fly to end things humanely.  Felt guilty when I saw the female on its own later that day.