Northampton and Wales wing George North should not play again this season, according to former World Rugby medical adviser Dr Barry O'Driscoll.
North was knocked out as Saints beat Wasps 52-30, the fourth time he has taken a blow to the head this season.
Northampton say the 22-year-old is being assessed and the club "will not be making any further statement yet".
Former Ireland international O'Driscoll said: "There should be no more rugby for George North this rugby year."
North was left motionless when he was struck on the head by Wasps forward Nathan Hughes's knee after scoring his second try for the Saints.
Hughes, who protested his innocence, was sent off for dangerous play.
Former Scarlets wing North also suffered blows to his head in the autumn and again during the Six Nations.
He missed Wales' 12-6 win over South Africa in November after being concussed against New Zealand, and was then struck twice on the head while playing against England during the Six Nations.
North missed Wales' next match against Scotland after being given an "extended recovery period".
O'Driscoll is an outspoken critic of the way rugby's governing body manages head injuries, and resigned from his post on its medical committee in 2012 in protest at the decision to trial a new protocol for dealing with concussion.
He believes North should take a break from the game, while having regular psychological, cognitive and physical assessments.
"Somebody who has four concussions in five months - two in one game - and is going back to such a brutal game as rugby should be having a long rest from rugby under the care of a neurologist," O'Driscoll told BBC Radio Wales Sport.
Northampton issued a statement on Saturday saying North would be assessed by a specialist this week and that the club "places the highest importance on player welfare".
Saints play Clermont Auvergne in a European Champions Cup quarter-final on Saturday, 4 April.