Spurs have kicked off talks with Nottingham Forest this week as they seek to find common ground to agree a deal for Johnson.
The north London side could offer fringe players as part of a deal, with Forest thought to want £50m for Johnson.
Postecoglou is convinced about Johnson and believe he is the perfect fit for his system at Tottenham, with the player attacking target number one.
It is suggested that the deal will be a tough one to do though and if it does happen then a breakthrough may only come towards the end of the deadline.
Nottingham Forest are not keen to lose Johnson, but could do business if the price is right.
Whether they would be prepared to sanction a late departure though is unclear as boss Steve Cooper is likely to want to reinvest the cash.
Johnson came through the youth system at Nottingham Forest and his sale would be counted as pure profit for financial fair play calculations.
