But Ryan has again got the nod to partner skipper Paul O'Connell who took little part in the pitch sessions this week as he nursed a gash on his knee.
Flanker Niall Ronan, who injured his knee against the Italians, was another on the easy list although he did train today in Musgrave Park.
From Munster's point of view, it is solely a question of getting the win they know will edge them closer to a place in the knockout stages of the Heineken Cup.
Depending on how things go in the Scarlets v Northampton match in in Llanelli, a victory on Saturday could secure qualification for Munster before they have to go to Milton Keynes.
If Munster win and the Scarlets lose, or if the Scarlets win without the bonus point, then Tony McGahan's men are through to the quarter-finals.
But Ronan accepts that as things stand, they are guaranteed nothing yet. "Of course the expectation is there among the supporters. We're at home, top of the group and all that," he said.
"But we know we still have to put in a performance. We don't know what sort of side they'll send over but we do know they're not going to come over and roll over.
"And it Castres get momentum and get their tails up, we know what they're capable of doing."
MUNSTER (v Castres Olympique): Denis Hurley; Johne Murphy, Keith Earls, Lifeimi Mafi, Simon Zebo; Ronan O'Gara, Conor Murray; Wian du Preez, Damien Varley, BJ Botha, Donnacha Ryan, Paul O'Connell (capt), Peter O'Mahony, Niall Ronan, James Coughlan.
Replacements: Denis Fogarty, Marcus Horan, Stephen Archer, Donncha O'Callaghan, Billy Holland, Tomas O'Leary, Ian Keatley, Danny Barnes.