Not even with other ducklings? I would try putting them back together after he/she can walk. For now though, just put it in a separate setup with some blankets and just keep an eye on him/her. You can add a mirror too, as long as it cant get broken. Keep us updated!
Do you plan on putting the duckling with the other ducklings? Just wondering- I have raised ducks with chickens as well and they do just fine!!