Ryan is playing for Munster 'A' against Leinster 'A' at Clonmel RFC on Friday.
Fogarty has ten Heineken Cup caps to his credit but Saturday afternoon's game will mark his first European start and while admitting it is a blow to be without Flannery, Munster coach Tony McGahan commented: "Of course it's disapponting, particularly so for the player himself but Denis has been around this squad a long time and fits in well.
"He's worked extremely hard over the last 18 months of his rugby career to get an opportunity to start a Heineken Cup game at home and I know he's thrilled and certainly looking forward to it."
MUNSTER (v Benetton Treviso): Paul Warwick; Doug Howlett, Lifeimi Mafi, Jean de Villiers, Keith Earls; Ronan O'Gara, Tomas O'Leary; Marcus Horan, Denis Fogarty, Tony Buckley, Donncha O'Callaghan, Paul O'Connell (capt), Alan Quinlan, David Wallace, Denis Leamy.
Replacements: Damien Varley, Julien Brugnaut, Darragh Hurley, Mick O'Driscoll, Niall Ronan, Denis Hurley, Peter Stringer, Ian Dowling.