I can imagine at the pre-match team talk the clown Millwall manager stressing that "Mateta is their danger man, Mateta must be stopped" with a big picture of Mateta on the screen just in case they weren't sure, and the message slowly seeped into the fukwit keeper's brain to the point that on the field of play he saw the opportunity to enact a fantasy of his.
What I see is deliberate foul play - their aren't many examples of a keeper charging out of his box, and then with his his foot at head height; it's not exactly out of the manual, and I hope that he gets a lengthy ban at the very least.