Ireland boss and former England coach Farrell revealed his worries on RFU plans for the English community game at Monday’s Guinness Six Nations launch in London.
Both Farrell and Ireland captain Johnny Sexton criticised the RFU move to impose a waist-high tackle limit from men’s level three and women’s level two downwards as soon as next season.
An online petition against the RFU changes launched by Old Reigatians fly-half Ed Bartlett has now amassed almost 70,000 signatures, as opposition to the stark rule-changes continues to grow.
“It’s super important that what has to come with that rule change is the correct coaching, and technique,” said Ireland head coach Farrell.
“If you’re just saying to a kid you need to tackle lower, then you become even more vulnerable in my opinion.
“You can be sitting ducks waiting for something to happen.
“The coaching and how it’s applied to tackling below the waist is absolutely crucial, otherwise we’re going to have a serious problem.”
The RFU’s snap announcement of the changes on Thursday followed the English union’s council voting in the changes on Monday.
In the rush to push the changes into the public domain however, the RFU opted to kick the details down the line.
A community game covering every level of the sport outside of professionalism will be drastically reshaped next season, leaving thousands of players expecting a radical difference in their chief sporting pastime.
Many anecdotes already abound of players pledging to quit the sport, with participation numbers always on RFU minds when it comes to the game’s future.
But as the law suits on head injuries continue to grow, rugby’s safety has never been more paramount.
Ireland skipper Sexton left no doubt over his dismay at the changes, which have already been trialled in France and could be implemented more widely at community game levels.
“I don’t agree with it, there’s no point sitting on the fence is there,” said Sexton.
“You’ve got tall people who play the game, it should be their decision on how to tackle.
“The tackles we really need to take out of the game are the reckless, sprinting out of the line, not wrapping arms.
“It should be an option to tackle a bit higher. It’s not like you can’t get concussed chopping someone’s knees.”
The RFU is now busy developing programs to detail and implement the new laws, with workshops and explainer sessions no doubt in the offing.
English chiefs must quickly clear up a clutch of grey areas however, especially focusing on the defence of pick-and-go attacks, second tacklers and limits around maul and ruck contact and binding.
Any nobility in the initiative will be swept away by poor implementation, and now the RFU must follow up both quickly and substantively or risk leaving plans open to continued challenge.