A potent try scorer from fullback or the wing, Folau was has been a linchpin for Australia's attacking prowess since 2012, when he made the switch from the AFL to rugby.
But without Folau, Erasmus said the Wallabies had lost their key strike weapon.
"They'll be less of a threat aerially without him … that is one thing you worry about," he said. "I don't know how influential he was as a leader but certainly I think we have a better chance in the aerial department.
"He is a great striker and finishes tries. He was the top-try scorer (in Super Rugby). He is an influential player. There is no doubt they will miss Folau because he is a world-class player but they interchange positions no matter the number on their back."
Despite the Folau issue, the Wallabies have not been written off with former New Zealand captain Richie McCaw admitting the side knows how to turn it on in the big games and warned the world to beware the wounded Aussies.
While he said it was "sad for rugby" to see Folau out, McCaw said the World No. 6 Australian side are primed to be World Cup dark horses.
"Knowing Australians like I do, they are competitive, hugely competitive and they will get themselves in good situation to compete," McCaw told Reuters. "They have got ability to win games now against everyone so you don't want to write them off at all.
"I think sometimes where people are not talking about them a lot leading up to (the World Cup), that's the best thing you can be. You just come in and surprise everyone.
"But from a New Zealand point of view, there is never a surprise that they front up."
With the World Cup set to take place in Japan starting September 20, the 2019 Rugby Championship is set to help tune the southern hemisphere powerhouses up for the event.
South Africa are set to field a second-string team for the tournament opener, although Erasmus said it's not a sign of disrespect for the Wallabies.
Only half a dozen of the line-up could be considered first-choice players, while a group of 15 have already departed for New Zealand to prepare for the Test against the world No.1 side in Wellington next weekend.
"There's no disrespect to Australia at all," Erasmus said. "If we didn't have the tough draw in terms of travelling with New Zealand and Australia we would have picked the best first choice team for this one and the next."
Australia have picked a strong team for the Ellis Park opener with Michael Cheika aiming to move on from the Folau saga.
The Wallabies last won at Ellis Park in 1963 and have since lost six times at the venue, with their most appearance a 53-8 drubbing in 2008.
Their last win in South Africa was in Durban in 2011, although Cheika's men managed a 27-27 draw in Bloemfontein in 2017.
Brumbies star Tom Banks has been given the first shot at the Wallabies fullback jersey after just three Test matches off the bench.
"I don't think you can deny Tom Banks on his form from Super Rugby," Cheika said. "He's been outstanding for the Brumbies and I'm sure he's going to do well.
"Beale finished the season quite well and he's been outstanding in training and to have a guy of that experience to finish for us is a big plus.
"He reads the game well, he knows where he wants to pop up and attack and he will work hard in defence. He's got good pace — there's going to be a lot of kicking threats from South Africa so he will cover that terrain in the back field very well."
There are four new faces in the side with Fijian-born Melbourne backrower Isi Naisarani set to make his Wallabies debut while NSW Waratahs rookie Harry Johnson-Holmes will be hopeful of getting on the park after nabbing a bench spot.
UK-based halfback Nic White and 86-Test veteran James Slipper will end their exiles from the national team. White has had four years out of the side, while Slipper has missed two, including a two-month ban in 2018 for cocaine use.
