Lealiifano and Bernard Foley are in the mix as coach Michael Cheika prepares to name his line-up on Thursday for the Rugby Championship match, which doubles as the opening Bledisloe Cup Test.
The pair have shared the duties this year with the Waratahs’ ice man Foley getting first crack in Australia’s opening loss to South Africa and Brumbies ace Lealiifano steering the ship in the win over Argentina in Brisbane.
Slack, who led the Wallabies on the triumphant 1984 Grand Slam tour and to a Bledisloe Cup series victory on New Zealand soil two years later, believes the call on who plays five-eighth is “single most difficult decision” facing Cheika.
Slack says the Wallabies don’t have the attacking brilliance of Kiwi playmaker Beauden Barrett or defensive starch of Ireland’s Johnny Sexton but they should back Lealiifano to guide them through to the tournament in Japan later this year.
“On the back of the two games and form throughout the year I would go for Christian because I thought he was better than Bernard,” Slack told AAP.
“In the 2015 World Cup Foley reached the absolute peak of his ability and was brilliant and I’m not sure we’ve seen it since.
“It’s an area where we’re not really fantastically blessed – they’re both good, solid international players but they’re not what some of the other teams have.”
With his line-busting prowess, Wallabies vice-captain Samu Kerevi appears to have a hold on the No.12 position but Slack would change his centre partner from Tevita Kuridrani.
“We need another ball player,” Slack said.
“You could arguably put James O’Connor at outside centre but I would probably go with Reece Hodge.”
A former selector, Slack believes Hodge has more ball-playing skills than Kuridrani, so better complemented Kerevi.
Kurtley Beale, who starred off the bench against the Springboks and started in the win over the Pumas, is Slack’s preference at fullback ahead of Tom Banks.
“I think you’ve got to pick Beale as we’re limited on game-breakers, as against the Boks he showed what he can do when he came on late,” Slack said.
“I just don’t think we’ve got the game-breaking quality in the backs at the moment to use him at our leisure with half an hour to go; he’s got to be there from the kick-off.”
“Koroibete is a weapon and needs to be there and I’m a Haylett-Petty fan,” Slack said.
Slack says the Wallabies needed to make the most of their ball if they were to have any chance of upsetting the All Blacks.
“Against the Pumas in the second half we had all the ball but couldn’t do much with it and if we repeat that k ind of performance we won’t give ourselves much chance,” he said.
“We need to play very, very close to our best to be realistic about beating them.”
Sign up to our mailing list here and we’ll keep you up to the minute with weekly updates from the world of rugby.