Horan had an ankle injury but came through training on Tuesday and if he does likewise on Thursday, the experienced prop will travel to France. Heineken Cup newcomer John Ryan, who took Horan's place last time out, has been retained.
Castres have ceded home advantage opting to play this weekend's encounter with Munster at the 18,754-capacity Stade Ernest Wallon rather than their own Stade Pierre Antoine, which has only house 10,000 spectators.
It is a busy week for Munster's frontline players and their second string, with Tony McGahan taking his senior squad to France and Ian Costello's Munster 'A' side will fly out the same day to Wales for their second British & Irish Cup outing against Neath at the Gnoll.
Following on from their narrow defeat to Cornish Pirates in Clonmel last weekend, Munster 'A' face a daunting task at the famous Gnoll against a home side who warmed up for this one with a 29-16 win away to title holders Bristol.
MUNSTER Squad (v Castres Olympique) - Backs (13): Conor Murray, Tomas O'Leary, Peter Stringer, Duncan Williams, Ronan O'Gara, Ian Keatley, Scott Deasy, Johne Murphy, Doug Howlett, Danny Barnes, Will Chambers, Lifeimi Mafi, Denis Hurley.
Forwards (17): Marcus Horan, John Ryan, Wian du Preez, BJ Botha, John Hayes, Stephen Archer, Damien Varley, Denis Fogarty, Donnacha Ryan, Paul O'Connell, Donncha O'Callaghan, Mick O'Driscoll, Billy Holland, Peter O'Mahony, Niall Ronan, James Coughlan, Denis Leamy.