"What happens on Tuesday, I'm not sure," Milligan said in Kazan. "But what I do know is that if Timmy's called upon, he'll be more than ready."
Australia's all-time leading scorer, with 50 goals in 106 appearances, Cahill is hoping to join the select list of players to have scored in four consecutive World Cups, following in the footsteps of players such as Cristiano Ronaldo and Pele.
But the 38-year-old has had to watch from the bench as the Socceroos' last-16 hopes were badly dented by a 2-1 defeat to France and a 1-1 draw with Denmark.
Bert van Marwijk's men now must beat Peru and hope France beat Denmark to have any hope of sneaking into the last 16 -- a feat they last achieved in 2006 when, under Dutchman Guus Hiddink, Cahill scored twice in a 3-1 opening win over Japan.
Former Socceroo Mark Bosnich joined growing calls for Cahill to play in what could be his final World Cup appearance.
"Timmy is a massive personality in the team, in the media, all over the place," Milligan said on Friday.
"But these decisions are not for us to make. We support each other, we know what he would do if he did get the chance, but that's not up to us to decide."
Cahill's chances appeared to increase when Urawa Reds front man Andrew Nabbout was ruled out with a shoulder injury following the draw with Denmark, after which midfielder Robbie Kruse received death threats from fans Down Under for a disappointing performance.
But with 19-year-old Daniel Arzani coming off the bench to underline his attacking threat against Denmark, Van Marwijk could now have a dilemma.
Milligan suggested Cahill's contribution did not only amount to what he adds on the pitch: "While I'm sure, undoubtedly, that he's frustrated that he's not had his shot yet, the amount of help he gives to the players is unbelievable."
Speaking of Arzani, Milligan added: "It's up to him over the next few days, as it is to all of us, to show Bert that firstly we deserve to start and secondly we're ready to start."
Asked about the abuse aimed at Kruse and his family, he added: "It's unfortunate. Some people forget that it's just a game.
"Robbie's been fantastic for a long time for us and I'm not the only one who thinks that. People are entitled to their opinion, but you never like to see it go that far."