The striker was on the wong end of a collision with Millwall goalkeeper Liam Roberts, who sent off for his action. Mateta was taken to hospital and later released on Saturday evening.
Parish said: "In all the time I've watched football, I've not seen a challenge like it. I looked to see how old the keeper was and he's 30 years old.
"That is the most reckless challenge on a football pitch that I think I've ever seen and he needs to have a long hard look at himself that lad because he's endangering a fellow professional - and maybe even his life with a challenge like that.
"It is difficult for me to talk about the rest of the game if you're worried about JP. It is a terrible challenge.
"They're professionals and they have to treat each other like professionals and have a duty of care for your fellow professionals. That keeper has not had one (duty of care).
"Credit to the players carrying on. It is difficult for me to think about anything else with JP in hospital. Why the referee (Michael Oliver) needed to go to the screen I have got no idea. He’s a human being. Forget he's a football player. It is not a normal challenge. We want to progress, but it is difficult for me to look past that challenge."