Two updates on this duck:
1) The other 3 ducks were obviously distressed after the attack and were not vocal, but after a couple of days they kinda got back to normal and I discovered that it was not Daisy that got attacked, it was her brother LuLu (LouLou maybe? We didn't know the sex when we named them lol)
2) I got him to drink a lot of the sugar water and even eat some of the "soup" and he showed some signs of improvement, such as standing up on his feet when I gave him drinks, but even then, he could not hold his head up and it would droop all the way down unless I held his head. My wife took him to the vet, and unfortunately, she said that there was irreparable neck damage, and the duck was never going to recover, so we had to let them put him down. I buried him next to the pond so he can be close to his brother and sisters.
RIP LouLou