Coach Michael Cheika announced on Sunday morning Waratahs duo Rob Horne and Jake Gordon would join the Wallabies squad to cover the injuries of Kerevi and halfback Nick Phipps.

Kerevi was injured playing for the Queensland Reds against the Auckland Blues in Samoa but will not require surgery in somewhat positive news.

The 23-year-old has been in terrific form for the Reds this year and would have had his eyes on a Wallabies jersey all year, given he was ruled out of the spring tour through injury as well.

"Samu has been a very instrumental player for Queensland," said Cheika on the ABC's Offsiders program. "He would have formed a big part of our leadership [group] so I'm going to replace him with a more senior player in Rob Horne, who has a lot of experience and he fills that role nicely.

"Rob has been a strong leader in our Wallabies environment and his experience will be invaluable in this relatively inexperienced squad.

"We really want to put a strong emphasis on leadership in this team and we have some young leaders in this squad who are continuing to develop."

The decision to bring back Horne is an interesting one given it appeared on Tuesday, when the initial squad was announced, Cheika was more than happy to leave any Europe-bound players out of Wallabies contention for this year.

Horne, Scott Fardy, Will Skelton and the retiring Dean Mumm were all left out of the squad despite being involved last year.

However, Cheika did insist he would call up any of those players if he felt they were good enough to be in the starting XV.

Horne has been playing at No.13 for the Waratahs but has not started a Test in the midfield since the second Bledisloe Cup match in 2012, five years ago.

He has come off the bench as a centre since then but traditionally been used by Cheika on the wing, as was the case against England in all three June Tests.

The four midfielders now vying for two spots are Hodge, Kuridrani, Horne and Kyle Godwin.

Hodge and Kuridrani combined in four of the five spring tour matches - excluding the France match when Hodge was rested - and this is a combination Cheika looks likely to stick with come Saturday.

Cheika is a big fan of Hodge and sees his best position as being No.12, so with Kurtley Beale out of action for the three Tests, Hodge is now the man with no Kerevi around.

Meanwhile, in the halfback department, Gordon is in a two-horse race with Brumbies No.9 Joe Powell to make his Test debut, following the news Phipps would take no part in the series.

Phipps injured his ankle in the Waratahs' 44-28 loss to the Highlanders in Dunedin and will complete his rehabilitation in Sydney.

The call-up for Gordon caps a meteoric rise after he replaced Phipps as the NSW starting halfback on a number of occasions earlier in the year.

"Jake was involved in our camp in Canberra earlier this year and has really stepped up for the Waratahs in Super Rugby," Cheika said. "He's another uncapped player but it will be good to expose him to the Test environment."

With Will Genia back in Australia and almost certain to start as the Wallabies halfback, Gordon and Powell will fight it out at training this week to secure a Test debut at AAMI Park.

The Wallabies arrived in Melbourne on Sunday at different times throughout the afternoon and will get their training week under way on Monday morning.