Adam Coleman will return home to Australia after suffering a knee ligament injury in the Wallabies' narrow win over Scotland on Saturday.
The lock picked up the problem, which tests have proven to be a grade two strain of the medial ligament, in the early stages of the 23-22 success at Murrayfield.
Coleman had also started the 32-8 hammering of Wales the previous week, but his participation in the tour is now over and he will miss further Tests against France, Ireland and England.
Wallabies coach Michael Cheika conceded the news was a blow, but has been impressed with the way Coleman has combined with Rory Arnold and taken on the responsibility of the line-out.
"It's bit of a shame for him because he's had a really good start to the year and he's worked his way through since June but in saying that it gives someone else an opportunity," Cheika said.
"There's two ways to look at it. There's discovery, so we've been able to find, get those two guys playing good rugby and starting to improve at this level and we've also got ourselves another very, very handy line-out caller and he's only in single digit Tests.
"He's got a good future, he's got to keep working on it, there's no doubt about that, he knows that.
"He's disappointed but that's the way footy goes sometimes, you've just got to go with it."