Bristol have confirmed centre Will Hurrell suffered a "probable stroke" after a Premiership match last month.
The 30-year-old was withdrawn with a head injury during the second half of the game at Leicester on 4 January.
Hurrell said he is "feeling better" but is suffering from "severe headaches, memory loss and panic attacks" in a Facebook post on Thursday.
"Player safety is of paramount importance," said Bears director of rugby Pat Lam in a statement.
"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.
"We are aware there may be a long road of recovery ahead for Will. He has the full support of his team-mates, management and staff."
After being taken off during the match at Welford Road, Bristol said Hurrell subsequently felt unwell and 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."
The Rugby Players Association praised Hurrell for his bravery, saying on Twitter: "We will continue to support you every step of the way."