Saturday's extraordinary 42-37 loss to the Wallabies at Twickenham, which followed an agonising 24-22 reverse against the All Blacks, was England's fourth straight defeat and fifth in six games -- albeit three were at the hands of New Zealand.
What made England's latest setback all the more frustrating was that they were 15-3 up early on following two tries by Chandler Cunningham-South.
But Australia, who lost five of their six games while finishing bottom of this year's southern hemisphere Rugby Championship, eventually prevailed in a match where the lead changed hands four times when replacement Max Jorgensen went over for a clinching try in the 84th minute.
That score came just minutes after it appeared Saracens lock Itoje's try had settled the game in England's favour.
"That wasn't acceptable and we will have an honest look at that," Itoje said of Saturday's defeat."There's no doubt we didn't want to be in this position. When we came into this autumn, we weren't expecting to lose our first two games. In times like this, it's tough.
But Itoje, speaking before the Springboks' game against Scotland on Sunday, added: "We're very disappointed, but it's still a tremendous opportunity against South Africa. We have the world champions coming to Twickenham. We have an amazing opportunity to do something.
"There's no doubt in my mind that if we do our thing, are more consistent and play smarter rugby, then South Africa is a game we can win...I want, this team wants, to do the shirt proud, to do the fans proud. Next weekend is an amazing opportunity to do that."
England coach Steve Borthwick lamented his side's increasingly slack defence against Australia as they departed from their pre-match game-plan.
"In general, we probably went away from what was good about us," said Itoje, with the second row adding: "We turned the ball over way, way too much (18 times). This team is at its best when we are direct, and we just need to be more consistent in regard to that.
"We need to be more aggressive in defence. We need to be more joined up. Because of the amount of turnovers, we probably weren't able to go as hard as we would like. The improvement will definitely be to go harder off the line."
