Wales' first of two Tests against Argentina saw a red card in the opening 30 minutes.
Juan Cruz Mallia was given his marching orders for a dangerous tackle on Wales scrum-half Kieran Hardy, with the Pumas full-back having chased after a high ball and levelled the Scarlet.
The collision, however, saw Mallia's head make contact with Hardy's face.
Referee Matt Carley checked replays of the incident - prompting gasps and jeers from the crowd - with the TMO, before brandishing a red card.
Carley said: "We've got clear head on head. I'm not seeing any reason to mitigate this.
"We're all agreed it's a red card."
Addressing the offending player, he said: "You come from a long distance, have a clear line of sight, there's no significant drop by the ball-carrier, you do not drop at the waist, it's not a significant dip."
Wales and Cardiff flanker Ellis Jenkins, commentating on S4C, said: "That looked really nasty.
"It's not malicious at all, but it's reckless.
"If you give 10 different referees that decision to make, they'd all make the same one, I think."
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