Skills in rugby union have not advanced in "30 to 40 years" and are "miles behind" other sports, says former Fiji sevens coach Ben Ryan.
Englishman Ryan, who led Fiji to 2016 Rio Olympic sevens gold, joined Welsh Rugby Union as a consultant in January.
Writing in a blog on his website, Ryan said the 15-man game will "become less accurate and based more on size and strength" if skills are not improved.
He warned that would result in a "dilution of the sport".
Rugby union suffers in comparison to US professional sports, according to Ryan, 45, who also worked with the National Basketball Association after leaving his role with Fiji.
"Take the NBA," he wrote. "Every attempted shot is technically on the money. Every single one.
"Watch them play or train and you will see the same exacting technique that has been practiced ad nauseam.
"You need to be technically brilliant in attack and defence or you get found out quickly.
"Skill reproduction is seen as a huge priority. Why? Because the teams at the highest level win the most. Skill equals success."
The former England sevens coach specifically criticised passing and tackling skills both at international level and in the Premiership, claiming that "pass technique varies with every pass" and "tackle technique [is] more about the hit than the process".
Yet Ryan added he was hopeful for the future development of the game, singling out Wales' first try in defeat to England in the Six Nations last weekend.
He added: "I did see two areas last weekend in the Wales vs England game. The first was the beautiful running lines created by Wales from their first phase move that created a great try.
"The second was the clever focus on the second man in the tackle looking to target the ball. It was a well-thought-out technique that gleaned results and showed inventiveness that all too often is a rare animal in the modern game."