And the Leinster boss also admitted it was prudent to rest the World Player of the Year Jonathan Sexton after the out-half suffered tightness during the week.
Jordan Larmour will start at full-back with Dave Kearney and Adam Byrne on the wings, while Ross Byrne starts at out-half.
“He just had a bit of tightness this week,” Cullen said of Sexton. “He played a lot of games in the Six Nations. He’s okay. It’s a little niggle. Somewhere on his leg, I’m not sure what it’s called.
“It’s prudent to rest him because we have some big games to come.”
Cullen stressed that all his squad must be prepared to face the prospect of missing out on big games as he sets his side’s sights on a remarkable double-double, after confirming that Rob Kearney was fit, despite being omitted.
“Rob Kearney is okay. I don’t want to talk about individuals because that is not what this team is about.
“It’s a difficult situation. The squad are away for a long time and then we come back together and there are a lot of tight calls.
“We’ve used 55 players and it’s a huge group and they’ve already got us to a PRO 14 semi-final. Our philosophy is everyone needs to step in.
“You hear coaches complaining about injuries unravelling their team and we don’t want to use that as an excuse.
“We’ve guys coming through the system and they can all contribute. It’s unfair to talk about guys missing out. It’s about a squad of players.
“This is the group we are and the way we want it to be. It’s the nature of the business, investing in the young players.
“Older guys know they need to facilitate that, it can be hard at times but we have to tell them this is the way it is this week. And when they deliver things in training there can be a next time.”
People will miss out on big days because of injuries or whatever else and there will always be tight calls. And we explain that to the players. We all have battles across the team.”