Jared Payne has been de-registered from Ulster's squad for the final two rounds of the Champions Cup pool stages.
The New Zealand-born centre has not played any rugby since the Lions meeting with the Chiefs on June 20 when he was struck with a headache-related problem.
The 32-year-old has been training with Ulster but has been suffering from recurrent headaches after sessions.
Speaking last month, Ulster director of rugby Les Kiss was optimistic that Payne would return to action in early 2018 but news that he has been de-registered from the Ulster squad for the crucial meetings with La Rochelle at Kingspan Stadium this Saturday and the trip to Wasps a week later.
Kiss was keen to say that Payne's welfare was Ulster's primary consideration.
He has been replaced in the squad by 20-year-old flyhalf Johnny McPhillips who made his debut in the inter-provincial defeat to Connacht over the festive period and impressed when making a substitute appearance against Leinster at the weekend.
Payne has been sorely missed by the province, especially in defence.
He now looks certain to miss the Six Nations also.