Ryan's turn will come, but Horan's inclusion gives the Munster bench added strength, the worth of which was seen towards the latter stages of that memorably encounter with Northampton last Saturday night.
Not taking away for one second what the starters had produced up to then, but when Denis Leamy and Donncha O'Callaghan entered the fray Munster managed to up their collective efforts against Saints.
Tomas O'Leary, another impact replacement, also reminded everyone of the quality player he is and Heineken Cup newcomer Will Chambers fronted up as if he had been playing for Munster for years rather than just a few weeks.
But the Munster players and management are fully aware that all the hype and back-slapping from last weekend will become a distant memory if they cannot back it up this weekend against Castres.
Tony McGahan stated: "Castres are a very good and a very consistent side. If you've been watching the Top 14 over the last two and a bit years, you see how they've progressed to being a top four side.
"They've established a very strong forward pack and the way their captain Chris Masoe has been playing the last few months has been terrific.
"They've a big forward pack, very strong, very physical, a set piece-dominated, very traditional French side. Then a strong 9 and 10 who kick the ball extremely well.
"(Romain) Teulet, who we know very well, is there to kick the penalties. Two big centres, (Romain) Cabannes and (Paul) Bonnefond, who are able to play through that channel one position, then play off the back of that. Impressive.
"They've had a wonderful start to this season, I know they came up short in the last few games but before that they'd beaten some top sides, Stade Francais, Toulouse, Biarritz. So we're meeting a quality side, no doubt about that."
MUNSTER (v Castres Olympique): Johne Murphy; Doug Howlett, Danny Barnes, Lifeimi Mafi, Denis Hurley; 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, John Hayes, Donncha O'Callaghan, Denis Leamy, Tomas O'Leary, Ian Keatley, Will Chambers.