The Ulster and Irish hooker suffered a hamstring injury during training on Tuesday and is in danger of missing the crunch game which will secure a place in the top tier Champions Cup next season.
There was no mention of Best’s issue during Wednesday’s media briefing, but afterwards it was confirmed he had suffered an injury at training.
Ulster are awaiting further scans on Thursday and will make a final call on the captain’s inclusion in the match day 23 on Friday.
Ireland head coach, Joe Schmidt, will also be monitoring the situation ahead of the summer tour to Australia.
Iain Henderson - who will miss Sunday’s game with Ospreys - is already a major doubt for the Three Test series down under and Best would be a further blow as the Grand Slam Six Nations champions look to prove their worth in the Southern Hemisphere.
Also out of contention for Sunday’s match at Kingspan Stadium are Matty Rea and Clive Ross, leaving Ulster light in the backrow.
It looks almost certain that Chris Henry - who played against Munster having been omitted from the squad for some time - will get further game time.
Robbie Diack, who will leave at the end of the season, will also be involved, again having been more of an onlooker recently.
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