The 31-year-old, who toured New Zealand in 2005, will be out of action for up to six weeks and will miss the Ospreys' RaboDirect PRO12 opener against Benetton Treviso on August 31.
But the No.8 is expected to be fit and available for the Welsh region's Heineken Cup kick-off against the same Italian opposition.
"The early prediction sees him back available around the six-week mark," said head physio Chris Towers.
"After undergoing investigation and bearing in mind where we are in the season, it was agreed that the best course of management was for Ryan to have some minor keyhole surgery."
The triple Grand Slam winner, who has won 67 Welsh caps, aggravated a previous knee injury during pre-season training.
The Liberty Stadium outfit lost 49-25 away to French side Clermont Auvergne on Friday. They next face Bath at home on August 18.