Socceroos vice-captain Mark Milligan's future under coach Bert van Marwijk at the 2018 World Cup will lie in the heart of defence rather than his usual midfield role.

The versatile Milligan has predominantly been used as a central midfielder in recent years but was told he will not be considered a midfield option for Russia and will only play at centre-back if he makes the final cut for the 23-man squad.

Case for the defence: Mark Milligan competes for the ball with Brazil's Jose Paulo. Credit:AAP

Van Marwijk's decision is the first step towards defining his new-look defence, which will be in stark contrast to that of his predecessor, Ange Postecoglou.

In going away from the back three employed under the previous regime, van Marwijk's back-four formation will likely see Milligan join Trent Sainsbury, Milos Degenek and Matt Jurman as the central-defensive options at the World Cup.