It is understood Seán O'Brien is losing his race against the clock to be fit to face Toulon in The Champions Cup.
Initially, the Ireland flanker suffered from a concussion against Wasps in Pool Five and was set on a path to follow the return to play protocols.
He soon developed an inner ear complication, known as vestibular impairment, which caused him to restart his protocols for the third time last week.
The Carlow man has not yet been cleared to return to the game and could miss out on playing against the club which pursued him last season before he decided to extend his contract at Leinster.
The highly impressive Josh van der Flier was promoted in his place for the eagerly awaited PRO12 derby against Ulster for which the 22-year-old flanker was the Man of the Match.
This was enough to convince coach Leo Cullen to retain his services for the encounter with the defending champions Glasgow Warriors, the game swept away by the weather at the weekend.
Should O'Brien be unable to resume in the number seven jersey he has owned for so long, Van der Flier has surely earned the opportunity to earn his first start in The Champions Cup.
Coach Cullen could be tempted to turn back to Ireland World Cup try-scorer Jordi Murphy, who has not been able to repeat the standards he set last year.
The multi-purpose loose forward has spoken about his desire to become a specialist in one position rather than a versatile option.
His specific skills make him more likely for seven than his other preferred option number eight.
For the moment, Van der Flier is the man in form and ready to take the step up into the highest company in European club rugby.