The Australia international is wanted by a number of clubs after his strong performances for club and country over the past six months.
Postecoglou is aware of the heightened interest in the player, including claims of a visit from a QPR scout next Tuesday, but issued a hands-off warning to any club looking to sign him.
"He's got two years to go with us (on his contract) so it's as much our decision as anyone else's," he told Herald Sun.
"With every good performance he has at national level I guess his value goes up for ourselves and for anyone else that wants him.
"He's very much a very important part of what we're about and what we want to create."
Meanwhile, Postecoglou confirmed that Melbourne have been in talks with former Fulham and Inter Milan midfielder Giorgios Karagounis.
"We have spoken to him and he has shown an interest," he said.
"It's still a little bit of a long shot for us, but he is someone who would definitely improve our squad and help us achieve what we want to."