The influential second row was forced from the field in Limerick with 10 minutes left of the 26-10 victory over the English outfit as Munster bounced back from an opening round defeat at Edinburgh.
It has now been confirmed that the 29-year-old old has strained ligaments in his left knee, although no mention has been made of any potential surgery to resolve the damage.
A 10-week absence would leave Ryan sidelined until the New Year, therefore definitely ruling him out of the next two installments of Heineken Cup action.
Ryan won't be fit to feature in the December double header with Perpignan, with the first tie at Thomond now just over six weeks away on December 8.
Head coach Rob Penney will hope to have Ryan back in the side by the time they head to Gloucester in Round 5 on the weekend of January 10, but that doesn't leave him with much leeway given that that fixture is only 12 weeks away.
Ryan will also be unavailable for Ireland's autumn Tests against Samoa, Australia and New Zealand next month.
There was more bad news for Munster from that crucial win over Gloucester, with full back cum wing Denis Hurley also picking up a knee problem at the weekend.
Hurley suffered medial ligament damage after replacing Felix Jones in the No15 shirt and is expected to be out of contention for five weeks, leaving him with an uphill battle to be fit to take on Perpignan.
Munster are already missing star wing Simon Zebo after the Lions tourist cracked a bone in his foot during the defeat at Murrayfield 11 days ago. Like Ryan, Zebo is looking at a 10-week period of rehabilitation.