Although Souness praised Mooy's goal, he was critical of Magpies midfielder Mikel Merino, who failed to close down the Australian when he initially played the one-two.
The former Newcastle manager told Sky Sports News: "How do you stop a goal like that? Quite easily! Mikel Merino, when he comes out to confront Mooy, his sole intention is to not receive any physical contact from Mooy, or indeed not to get hit with the ball.
"It's a joke! You're defending 20 yards from your goal, take a blow for the cause. You can't win games of football when you've got people making those decisions so close to your goal.
"It's weak, it's like something you'd see in kids' football. You've got to stand back and admire the goal, but from the other side of the coin, you've got to speak to him. Take a blow for the cause!"
Rafael Benitez's charges are still looking for their first Premier League points of the season after the defeat.