22-year-old Johnstone, who came through the youth ranks at Old Trafford, will set the Hoops back £1million and United may sell the young stopper when his loan spell at Preston ends later this month.
The Preston-born goalkeeper has placed 23 games over two spells for his hometown club, and helped them to promotion last season from League One via the play-offs.
Capped at England youth level, Johnstone is yet to make a first-team apperance for United but has enjoyed loan spells at Scunthorpe United, Yeovil Town and Doncaster Rovers.
The Hoops are lining up the move as Green’s days at the club are looking numbered.
One of the biggest earners at Loftus Road, the 35-year-old will see his contract extended should he play 30 games for QPR this season and he is just five games away from seeing that happen.
The West London outfit are keen to get the England international off the wage bill, and Premier League sides Bournemouth and Watford are monitoring the situation.
QPR have struggled to adapt to life in the Championship following their relegation from the top flight last season, and currently sit a disappointing 15th in the table.
What do you make to the news QPR fans? Think your club should get rid of Green? Would Johnstone be a good replacement? Let us know in the comments section below!