At the moment, I am leaning toward abrasion, but?
What kind of perches does he have available (Dowel, Custom (metal), Natural Wood Branches, Specialty Branches, etc...) and what sizes are they?
Does he have any sandpaper or concrete perches?
Does your MAC commonly perch (sit) low especially when roosting?
In looking closer at the wound, it appears like it could have possible split /separated or is being pulled apart (possibly Brumblefoot). And abrasion or injury based wound would have a slightly different appearance along the edges.
How long has he had this wound. By the way how long have your had this MAC(?)? How old is he and when was his last visit to his Avian Vet?
Regardless, a wound like this needs to seen and treated by an Avian Vet. If infection sets in, he could lose his leg.
See your Avian Vet and the sooner the better, like today or tomorrow would be nice.