Scans have since revealed the full extent of the damage to his cruciate ligaments and confirmed the club's worst fears that Bryan will require surgery. It is now estimated that he could be out for up to seven months.
Rhys Oakley came in for last weekend's Celtic League defeat at Connacht though Scotland international Ross Beattie is also available to coach Chris Anderson.
He said: "This has been a difficult month. Autumn internationals and injuries have stretched us this week and we had the unfortunate news on Richard Bryan which is a blow as he has been playing well."
New Wales cap Luke Charteris, who made his international bow during the Autumn Series, is also battling to recover from a toe injury.