Okay...that looks like the talons of a large owl...if it were canine, there would be opposing punctures, at least one on the other side of the cat. Secondly, a canine would have carried him without dragging him in the grass.
Owls have feathers that make them silent when they dive and glide...they grab from behind with the talons and keep going. I have seen owls grab and successfully carry for a short distance, cats, rabbits and small dogs. I actually grabbed my mother's dachshund out of the talons of a great horned owl in her front yard one night. They hook their large center talon in and curl the foot to hold so usually you see just 2 punctures. Most likely Sam struggled and caused the owl to try to regain its grip and it could not stay aloft with the cat and Sam slipped away...hence the tearing of the wound. His weight and the nature of an owl attack explains the green stains...Owls strike from just barely above their target and in a forward motion...Sam would have been hit and dragged a few feet while the owl tried for lift carrying his weight.
There is not much you can do to stop that predator...they are Federally Protected, danged near impossible to ward against and they are silent and smart. Good luck with Sam, but I bet if the wounds were cleaned thoroughly and kept clean, he will be fine.