World Rugby wants more information from the Rugby Football Union about Northampton's "apparent non-compliance" with concussion protocols relating to George North's latest head injury.
The Northampton Saints wing should not have returned to the field of play after a mid-air tackle on 3 December, an RFU concussion review panel said.
But the panel determined Saints will face no sanctions over the incident.
The Saints medical team have accepted that North may have lost consciousness.
The 24-year-old Wales international previously suffered four head blows in five months between November 2014 and March 2015, leading to a spell on the sidelines that lasted from 27 March until 29 August.
The panel's 17-page report into his latest injury, released on Wednesday, stated that the head injury assessment (HIA) protocol was followed correctly.
But the game's governing body says that assessment should not have been undertaken and that the Wales international should have been "immediately and permanently removed" because of "a suspected loss of consciousness" involving a "high-risk player based on medical history".
It acknowledged "a risk of human error" but said it was "disappointed" at decisions taken that led to North remaining on the field of play.
Speaking after Friday's win over Sale, Saints boss Jim Mallinder said he was aware of World Rugby's statement but it was one for the RFU to consider.
On North's fitness, Mallinder added: "George is training with the squad. He is mentally fit and physically fit and, when everyone is happy, he will return to playing, which he is looking forward to."
What did the panel say?
The Concussion Management Review Group, which spent two weeks analysing the case, also concluded that North should not have returned to the field.
But it opted not to recommend any punishment - saying Northampton did not fail any protocols or ignore North's best interests.
North appeared to lie motionless after the incident, but told medics he had stayed still because he was "concerned about his neck".
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The lack of any sanction against Northampton has prompted severe criticism.
Dr Barry O'Driscoll, a former World Rugby medical adviser, suggested rugby union's authorities are "experimenting on players' brains" by failing to address concussion.
The Rugby Players' Association (RPA) said North's return to the field was a "significant failing", and believes sanctions would have sent a "clear message" about the "gravity of concussion management".
Former Scotland international John Beattie said he was worried for the future of young players, adding: "We can't have a game where the end product is a brain-damaged super-human who's made a bit of money."
Peter McCabe, chief executive of brain injury association Headway, said "serious questions have to be asked" of the existing protocols, adding: "This incident sends out a confusing message around the issue of concussion, particularly for children who follow the example of famous players and favourite clubs."
Sports injury lawyer Ian Christian said the panel's decision was "hugely disappointing", and a missed opportunity for rugby authorities to "make a statement".