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I'm a big believer that the Wallabies need two bigger ball runners/power defenders at 12 and 13. Quade Cooper's passing is good enough that 12 doesn't need to be a ball player, especially with Beale and O'Connor is the back three and able to pop up when necessary. With those two there, oppositions can't afford to pressure Cooper out of the game. I noticed a couple of tests last year where Cooper would throw his little pop passes to guys who weren't big enough to make the most of the few seconds of defensive uncertainty (it was Giteau and Barnes I think). A bigger guy would also ensure the midfield defence a bit- which would be absolutely essential against the likes of England in a RWC match.
Although I severely detest one Mark Gerrard, I wouldn't mind Deans thinking long and hard about a AAC/Gerrard, Gerrard/AAC, Mortlock/AAC, AAC/Mortlock midfield. Gerrard's main weakness, his speed, is less of a problem closer to the ruck and if the game turns into a kick-a-thon he is a good go to guy for tactical kicking. Ioane would be an option at 13 too, however with Mitchell out we need him in the OBs.