That happens at our place all the time. They (the dogs/cats) usually finish the job, but once last year I found one that was similar to yours and still alive so I took it to our local vet and he euthanized it for me for five bucks. I wouldn't let the poor little guy suffer anymore.
Stella - sweet, timid, elegant, lovely, lively, amazing
Luna - large, unattractive, naughty, adored