Pocock, who was playing number eight, came off the field late in the Wallabies' 39-28 loss at Lang Park and scans early on Sunday confirmed the extent of the injury."Pocock has unfortunately got a displaced fracture in his eye socket so he'll be out four to six weeks or something like that," Cheika told reporters at Brisbane airport.
"He played very well last night. He came into a cleanout and must have taken some contact there as he's got a nice little cut as well. He didn't really feel it until after the match."
The loss of the 28-year-old is a major setback for Cheika as he bids to square the series in the second of the three tests in Melbourne next Saturday.
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Usually an openside flanker, and one of the best poachers of the ball in world rugby, Pocock has formed an innovative back row partnership with Michael Hooper and Scott Fardy over the last year.
"We're obviously disappointed to lose a player of David's quality," Cheika added.
"Obviously (we'll miss) the skills he brings to the game and that dynamic we have in the back row with Michael and Scott but we'll probably create a different type of complementary nature now."
Cheika said the decision on who replaces Pocock in the side would come down to whether they retain the "twin opensides" back row or revert to a more traditional formation with a big ball carrier at number eight.
Fortunately for Australia they have great depth at openside flanker, with Sean McMahon having a good game off the bench on Saturday and Liam Gill offering a top quality option as back-up.
Wycliff Palu and Ben McCalman would fit the bill if he decided to go with a more traditional number eight, Cheika said.
"We'll have another look at the game today and see how we want to play next Saturday," he added.
"If we think we should keep it, we'll go like-for-like, if we want to change we might do something different."
While Pocock will stay in Brisbane for treatment, lock Rob Simmons, Australia's lineout organiser, will travel to Melbourne despite lasting just 25 minutes on Saturday before coming off the field with a bad back.
"He's coming with us, he's had a good improvement from yesterday to today so we'll see how that looks tomorrow," Cheika said.
"Right now we're hopeful he'll be available."
(Editing by Peter Rutherford)
