Bristol centre Will Hurrell suffered a "probable stroke" following last month's game at Leicester, the Gallagher Premiership club have confirmed.
The 30-year-old suffered a head injury during the second half of the Premiership game on 4 January and was later admitted to hospital after falling ill.
Hurrell, who it was felt had originally suffered concussion in the incident, posted on Facebook: "I am feeling better but still (have) severe headaches, memory loss and panic attacks."
A statement on the Bristol website said: "Subsequent investigations throughout the past month now suggest that Hurrell suffered a probable stroke."
Bears director of rugby Pat Lam added: "We've been told by the specialists that Will's case is an extremely unusual presentation, including the part of the brain that was affected.
"Given the rare nature of the event, we have been treating the situation with utmost caution to ensure the best outcomes for Will.
"Player safety is of paramount importance and we are aware that there may be a long road of recovery ahead for Will.
"He has the full support of his teammates, management and staff."
After being taken off during the match at Welford Road, Bristol said Hurrell was treated at Leicester Royal Infirmary.
Hurrell said: "It's been a tough six weeks but the support I have received has been unbelievable.
"I do not know what the future holds but I am feeling positive whatever that is."