Sam Cane would have more likely have been yellow-carded for a accidental high shot on Robbie Henshaw which saw the Irishman stretched from the field.
Former England international and Harlequins defence coach Nick Easter said on Twitter the new rule will bring diving into the game of rugby.
"Goodbye to the game as we know it! Enter play acting/diving to punish tacklers. Madness and bedlam will ensue," the 54-test veteran tweeted.
The zero-tolerance approach follows research from 2012-15 of more than 600 head injury assessments in 1,516 elite-level matches around the world. The research showed 76 percent of all head injuries occurred in tackles, an injury to the tackler was 2 1/2 times more likely than to the ball-carrier, and tackle height was a contributing factor.
Former England international and commentator Stuart Barnes said the change had to be made the save the future of the game.
"Coaches have got into the culture of hits, players have got into the culture of hits. Players are stronger, they have better conditioners so at what stage of this power arms race for powerful players going to stop because if it doesn't there's going to be an accident and someone's going to have something terrible happen to them," he told Radio Sport Breakfast.
"We're going to be in a state where people are not going to want to play rugby because it is just too fierce and too ferocious. That is an issue. If it means every now and then someone gets a red card because they hit the shoulder and rides up then I think that's right."
The law change will come in effect from January 3 next year.
World Rugby Chairman Bill Beaumont said: "World Rugby continues to be proactive in aligning with the latest evidence-based recommendations in this priority player welfare area to ensure players and coaches at all levels of the game are appropriately educated, managed and protected when it comes to head impacts and injury within the environment of a contact sport.
"We believe that we are playing a leading role in terms of the development and implementation of best-practice interventions and this important study further reflects our commitment to an evidence-based approach to player welfare. We believe that the invaluable data from this study will inform the law review process and lead to changes in playing or training practices."
However, All Blacks and Blues hooker James Parsons doesn't think the rule change will have much effect on how the players approach the tackle area.
"In terms of the way we're coached, that's where they want you to tackle anyway," Parsons told the Radio Sport Breakfast.
"I don't think a hell of a lot will change. I just think it means they'll ref it a lot harder if you do have a slip-up and probably be reprimanded a little bit more firmly if you have to in front of the judiciary.
"Most of the time you see guys aiming for the shoulder and when it goes wrong is when they're the second guy approaching the tackle where they are normally trying to finish the tackle off or trying to get in close to the ball and rip it out."
THE NEW RULES:
Reckless tackle
A player is deemed to have made reckless contact during a tackle or attempted tackle or during other phases of the game if in making contact, the player knew or should have known that there was a risk of making contact with the head of an opponent, but did so anyway. This sanction applies even if the tackle starts below the line of the shoulders. This type of contact also applies to grabbing and rolling or twisting around the head/neck area even if the contact starts below the line of the shoulders.
Minimum sanction: Yellow card
Maximum sanction: Red card
Accidental tackle
When making contact with another player during a tackle or attempted tackle or during other phases of the game, if a player makes accidental contact with an opponent's head, either directly or where the contact starts below the line of the shoulders, the player may still be sanctioned. This includes situations where the ball-carrier slips into the tackle.
Minimum sanction: Penalty
