Like most watching, Steve Parish was left shocked by a high boot from Millwall goalkeeper Liam Roberts.
Roberts was sent off following a VAR review in the eighth minute after rushing out of his goal in bizarre fashion.
However attention quickly turned to Mateta, who required extensive treatment before being taken to an ambulance in a stretcher.
Giving the BBC an update at half-time, Parish said: "He's got a bad gash behind his ear and a head injury, he's at the hospital. We hope for the best.
"You know, there's a lot of emotion in football, but we need to talk about that challenge. In all the time I've watched football, I've never seen a challenge like it.
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"I looked to see how old the keeper was and he's 30-years-old.
"That's the most reckless challenge on a football pitch, I think I've ever seen."
Continuing his condemnation, the Palace co-owner said: "I think he needs to have a long, hard look at himself that lad. He's endangering another professional with a challenge like that.
"It's very difficult for me to talk about the rest of the game because we're worried about JP and it's just a terrible, terrible challenge.
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"They're professionals and that's another thing, they're all professionals out there and they've got to treat each other like professionals.
"They have a duty of care towards their fellow professionals and that goalkeeper hasn't had one. Credit to the players for carrying on.
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"It's difficult for me to think about anything else with JP in hospital.
"As I say, in all my time watching football, I think someone mentioned there was one in the World Cup in 1982 like it, but it's a dreadful challenge.
"And why the referee needed to go to the screen, I have no idea."
More to follow.