Ace flanker David Pocock has been ruled out of Australia's key World Cup pool match against Ireland with a back injury, team officials said on Saturday. Pocock missed Friday's final Captain's Run at Auckland's Eden Park and failed to recover sufficiently from a back strain to play against the Irish later Saturday. He will be replaced by Western Force team-mate Ben McCalman, who comes off the reserves bench and into the starting XV to play his 15th Test, but his first as an openside flanker. Pocock experienced soreness in his lower back which has previously troubled the star loose forward. A team spokesman said Pocock reported less discomfort on match morning but the improvement wasn’t considered enough to enable him to play. While the Wallabies face a match which will be critical in determining the winner of Pool C, coach Robbie Deans said he was not prepared to take any risks with Pocock, the only specialist open-side flanker selected in the 30-man Australia squad. "It’s more of an on-going irritation than anything serious, scans have shown nothing sinister, but David is a key member of our group," Deans said. "We would rather give him a bit of time to get it right before putting him back out there." With McCalman to start the match, NSW Waratahs number eight Wycliff Palu has been added to the bench, with Deans opting to retain a five-forward, two-back split among his run on substitutes. "Although Ben isn’t a recognised openside (flanker), he does have some recent background there and has trained in the role on a regular basis with us this year," Deans said. "We were looking to give him an outing on the openside before our Pool programme concluded, it’s now just come a bit earlier than we’d originally planned. "He is well suited to the role, being strong around the contact area and good over the ball, which is a legacy of his time in the Australian Sevens programme. "He’s looking forward to the opportunity and we believe in him. That’s why we opted to utilise him as David’s back up when we looked at our selection mix for the squad." Should Palu play against the Irish, it would be his 37th Test appearance, but his first since Australia beat Wales 33-12 at the end of its 2009 tour of Europe. Revised Wallabies team: Kurtley Beale; James O’Connor, Anthony Fainga’a, Pat McCabe, Adam Ashley-Cooper; Quade Cooper, Will Genia; Radike Samo, Ben McCalman, Rocky Elsom; James Horwill (capt), Dan Vickerman; Ben Alexander, Stephen Moore, Sekope Kepu. Replacements: Tatafu Polota Nau, James Slipper, Rob Simmons, Wycliff Palu, Scott Higginbotham, Luke Burgess, Drew Mitchell.