The 28-year old midfielder was the subject of a bid from Premier League club Crystal Palace recently but Postecoglou says he has heard nothing from the Eagles since and is confident he will be able to keep the Australian international in Melbourne.
"Anything can happen till the end of the transfer window but as far as we are concerned Milligan is far too important for us to lose," Postecoglou said.
"He is so important that he is almost irreplaceable.
"People might assume that Milligan would be disappointed at missing out on a dream transfer but it is not the case.
"Milligan is playing for a club that has 20,000 members and at the end of the season, where he should play regularly, he has a World Cup to look forward to.
"We are not a consolation prize.
"For Milligan it is not about money or profile. He loves the club and he's happy here and I have seen nothing in his attitude to suggest otherwise.
"He did well for us last season and I think he will do better this time because I think there is improvement in him.
"He is now an important part of the national team so replacing him is not easy."