He has played only a handful of Super 15 and test matches in that role, but is in the World Cup squad and starts tomorrow at Suncorp Stadium against the All Blacks. McCabe has played most of his top-flight rugby at No 13, wing or fullback.
He was as stunned as anyone when Brumbies assistant Stephen Larkham - who knew a thing or two about changing positions and starring in test footy - sidled up and told him he was playing second five-eighths.
"He spoke about how dangerous the Reds backline was so he wanted them to make more tackles in the frontline. We wanted to be direct so I got a lot of carries and it worked a bit," McCabe recalled.
"Things are starting to come more instinctively but I have to get used to others and how they play."
At Eden Park he and the Wallabies were shut down by a strong All Black performance but are aiming for revenge tomorrow against the visitors in Brisbane. Ma'a Nonu and Conrad Smith were a strong combination in Bledisloe I, helped by the quick ball and space the Wallabies gave them in getting off their line.
"They are two great players and two massive threats that we have to worry about regardless," McCabe said. "If we rush, their footwork is too good so we have to be more connected with better numbers across the park."
McCabe loves the steep learning curve he has been through in his test education and clearly Deans admires what he sees.
Advice comes from the coach, plenty of video analysis and long chats with Cooper and Berrick Barnes. Being a frontline defender suited his psyche and now he has had to sharpen his distribution skills.
How did he get used to Cooper? McCabe was unsure anyone could get used to the lively Reds playmaker - but he said the beauty of the inside-back arrangement was that if he got into space, Cooper would find him.
"My role is either getting the gainline or holding up the defenders to give the others as much space as possible because of the danger we have further out."