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The controversial incident happened late in Everton's 2-0 win at Anfield on Sunday when Dominic Calvert-Lewin collided with the sliding Alexander-Arnold - first connecting with the defender's head, earning a penalty.
Some viewers didn't believe it was even a penalty but Clattenburg is sure the decision was right and claimed the Liverpool right-back "could have been dismissed".
To be clear, Clattenburg - writing in his Daily Mail column - agreed that common sense suggests that Alexander-Arnold didn't deserve a red card.
But Clats was wondering whether, by the letter of the law it should be, as he compared it to Arsenal's David Luiz being sent off against Wolves earlier this month.
Liverpool fans, far from convinced it should even have resulted in a spot-kick, had their say on Twitter.
But Clattenburg's view in the Mail was that when on-field referee Chris Kavanagh went to the monitor to check the incident, it wasn't to decide on the penalty; it was to assess whether Alexander-Arnold should be red carded.
"Dominic Calvert-Lewin had an open goal, so there was no reason for him to dive," he wrote. "As he hurdled Trent Alexander-Arnold to get to the ball, the Liverpool full-back lifted his head and created contact.
"It was a penalty, but it was good to see common sense prevail and Alexander-Arnold avoid a red card.
"The 22-year-old could have been dismissed for the denial of an obvious goal scoring opportunity with no attempt to play the ball, if referee Chris Kavanagh had followed the same principles which saw David Luiz shown red at Wolves."
The former Premier League referee explained: "Kavanagh spent mere seconds at his monitor. It wasn't to check whether it was a penalty - he and VAR Andre Marriner accepted it was - but whether Alexander-Arnold should be red-carded.
"Kavanagh applied the spirit of the law and decided not to further punish Liverpool. To avoid any more incorrect reds like the one for Luiz, the law should be changed."
Do you think that - according to the current laws - Trent was lucky not to be sent off? Let us know in the comments.